Position 2 oven racks in the upper and lower thirds of the oven and preheat to 350 degrees F. Line 2 baking sheets with parchment.
Combine the butter, brown sugar and confectioners' sugar in the bowl of a stand mixer fitted with the paddle attachment and mix on medium speed until fluffy, about 2 minutes. Scrape the paddle, sides and bottom of the bowl with a rubber spatula, then add the peanut butter and beat on medium until smooth, about 1 minute.
Add the egg and mix on medium speed until combined. Add the bourbon and vanilla and mix on low to incorporate. Scrape the paddle and bowl with the rubber spatula.
Add the flour, baking powder, baking soda and salt and mix on low until just combined. Scrape the paddle and bowl with a rubber spatula. Add the oats and chocolate-covered pretzels and mix on low until just incorporated. Scrape the paddle and bowl with the rubber spatula to make sure the dough is uniform.
Using a 2-ounce ice cream scoop or 1/4-cup measuring spoon, scoop 6 mounds of dough onto each of the prepared baking sheets. Place 1 square of chocolate on top of each mound.
Bake until the cookies have spread and are lightly browned around the edges, 16 to 18 minutes, rotating and switching the positions of the baking sheets after 8 minutes.
Remove the sheets from the oven and immediately mist each cookie with 2 or 3 pumps from the spray bottle of bourbon. Let the cookies cool on the tray, about 15 minutes.