Add the room-temperature butter, brown sugar, and granulated sugar in the standing mixer bowl. Mix until combined, then turn the speed up on the mixer and whip until lighter in color and texture, about 1-2 minutes.
Add the peanut butter, mix until fully combined.
Add the egg and vanilla and mix until combined.
Pour the dry ingredients on top: flour, baking powder, baking soda, and salt. Turn the mixer on low and mix until the dough forms.
Remove the dough and wrap in plastic wrap, place in the fridge to chill for 1-3 hours.
Preheat the oven to 350 degrees. Remove the dough from the fridge.
Scoop the dough with a regular size cookie scoop into a small bowl of sugar. Roll the dough until fully covered, then place it onto light metal cookie sheets lined with parchment paper.
Use a fork to make a crisscross on top of the cookies.
Bake for 12-14 minutes.
Remove and place a Reeses on top of each cookie while still warm.
Melt the milk chocolate in the microwave 30 seconds at a time, stirring thoroughly in between each time.
Using a small plastic sandwich bag, cut a small piece of corner off and spoon the melted chocolate inside.
Using the small sandwich bag like a piping bag, carefully make 4 lines on each side of the Reeses, making 8 spider legs.