Combine the water, corn syrup, sugar, and butter in a 3-quart stainless steel pot. Split the vanilla bean lengthwise with a paring knife and scrape out the seeds, adding the seeds to the pot. Place the pot over medium-low heat and cook, gently stirring until the syrup is bubbling vigorously around the edges, about 5 minutes. Raise the heat to medium-high, clip on a candy thermometer if you like, and cook, without stirring, until the syrup is golden brown, about 10 minutes (or approximately 340°F). I didn't find it was necessary to use a thermometer - once the syrup starts to brown, it will be ready in a couple of minutes, so keep an eye on it. The edges of the syrup may brown quicker than the middle - if this happens, you can swirl the pot to distribute the sugar. The syrup will be ready when it is the color of smooth peanut butter - put a can next to the stove to compare.

While the sugar is cooking, mix together the baking soda and salt and have it ready to go by the stove. Grease an aluminum baking sheet, or put down a piece of parchment paper. 

When the syrup is golden brown, turn off the heat, remove the thermometer (if using), and stir in the baking soda and salt with a heat-resistant spatula. Be careful - the syrup will be incredibly hot! Stir in the peanuts, then scrape the mixture onto the prepared baking sheet and spread into a roughly 12-by-8-inch layer. Don't worry if you end up with large clumps of peanuts stuck together - you can break them apart later if you like. Cool to room temperature, about 1 hour.

Chop or break the brittle into 2-inch pieces. And, voila! Wasn't that easy? Now you have delicious caramel-vanilla peanut brittle that is an amazing candy in and of itself. Store half of the brittle in an airtight container - it will keep for about a month at room temperature, or up to 6 months in the freezer.

Coarsely chop the remaining half of the peanut brittle. Put the pieces in a food processor and pulse until the pieces break down into a loose powder. Add the peanut butter, vanilla, and salt and grind to a smooth paste, about 2 minutes. It will start off looking like a ball of dough, but keep the processor going until the paste loosens and smooths out into a thick but spreadable consistency. Stella Parks recommends you transfer the filling to a piping bag fitted with a 1/2-inch plain tip, but I didn't have one and just placed the filling in a bowl and dolloped it onto the cups with a spoon. This also worked fine, although maybe piping it would ensure a more even and consistent layer of filling. I give instructions for both methods later.

Temper the chocolate. Coarsely chop about 2/3 of the chocolate into 1/4-inch cubes, and place into a stainless steel bowl. Finely chop the rest of the chocolate and set aside. Heat a saucepan containing about an inch of water over medium-low heat until the water is just barely simmering. Place the metal bowl with the chocolate on top of the saucepan to create a double boiler, making sure that the bottom of the bowl doesn't touch the water. Stir the chocolate to melt it, while monitoring the temperature with a digital thermometer - do not let the temperature get above 105°F. If the temperature is rising too rapidly, take the chocolate off the heat for a few seconds and stir to release heat. When the chocolate is completed melted and just shy of 105°F, about 8-10 minutes, take it off the heat and stir in about 1/3 of the finely chopped chocolate. Stir until all the chocolate has melted, and then add in the remainder of the finely chopped chocolate, except for about 1/4 cup - save this 1/4 for emergency use. Stir for 5 minutes, until all the chocolate has melted (keep stirring even if the chocolate melts before the 5 minutes - this is necessary to properly agitate the chocolate). After stirring, the goal is to have the chocolate be at 84-86°F - it is now ready for use! If the chocolate cools too quickly and becomes thick, place the bowl over the hot water bath for 5 seconds, then stir vigorously off heat.

Notes: To test the chocolate, dip a butter knife into the chocolate and refrigerate for two minutes. If it comes out soft or melts to the touch, it hasn't tempered correctly, and your best bet is to repeat the whole process. If it comes out streaked or swirled but semi-firm or slightly tacky, it needs to be agitated more - stir vigorously for two more minutes, and test again. If it comes out hard, glossy, and snappy, it's tempered and your chocolate is ready for use!

During use, if the chocolate cools and becomes too thick, carefully rewarm it by setting it over the water bath for a few seconds, then stirring off heat. However, do not let the chocolate exceed 86°F - if it accidentally overheats, immediately add the "emergency" chocolate and stir like mad.

Assemble the peanut butter cups. Line muffin pans with fluted cupcake papers. I found it helpful to make the cups in batches of 6 or 12. Drop a tablespoon of chocolate into the cupcake papers. Add the peanut butter filling one of two ways: 1) With the piping bag, hold it vertically, perpendicular to the layer of chocolate, and pipe about 1 tablespoon of filling into the center of the chocolate. 2) Using a spoon, gently drop about 1 tablespoon of filling into the center of the chocolate. In either case, dropping the peanut butter filling should push the chocolate up the sides of the paper. Top each cup with another tablespoon of chocolate, smoothing the tops if necessary with an offset spatula. Lightly tap the pan on the counter a few times to even and smooth out the tops further. Refrigerate until the chocolate is set, about 30 minutes. Repeat with remaining cupcake papers, chocolate, and peanut butter filling.