1/2 cup (113 grams) unsalted butter, room temperature
1/2 cup (100 grams) light brown sugar, packed
1/4 cup (55 grams) granulated sugar
1 large egg, room temperature
1 teaspoon vanilla
1 teaspoon brewed coffee (leftovers from the morning is fine)
1 cup (110 grams) all-purpose flour
1/4 cup (30 grams) Dutch-process cocoa powder*
1/2 teaspoon aluminum-free baking powder
1/4 teaspoon salt
1 1/2 cups (312 grams) Reese's Pieces, divided
Preheat oven to 375F/190C. Line two baking sheets with parchment paper.
Sift flour, cocoa powder, baking powder, and salt into a medium bowl.
With an electric mixer, cream butter and sugars in large bowl until fluffy. Add egg, vanilla, and coffee, mix to combine. Add dry ingredients and mix until just blended.
Add 1 cup of Reese's Pieces to cookie dough, stir with wooden spoon to disperse.
With two small spoons or a small ice cream scoop, drop dough 2 inches/5 cm apart on baking sheets (11-12 per sheet, 22-24 total).
Bake 7 minutes, then top with 3-4 candies and press into the partially baked cookies. Bake for another 2-4 minutes until just baked through. Cool on baking sheets. Store in airtight container with waxed paper or parchment between layers of cookies.
*Cocoa that has been "Dutched" has neutralized the acids in the cocoa. Baking soda relies on an acid to perform as a leavener (ever mixed baking soda and vinegar as a kid?), so baking powder must be used as leavening with Dutch process cocoa, as it simply needs moisture to do its job. It's difficult to find Dutch process cocoa in the US; I order mine from Amazon.
If you can't find it, or don't want to purchase it for just one recipe (though if you like hot cocoa - the real deal - it makes amazing hot cocoa), go ahead and substitute regular natural cocoa powder, and baking soda in place of the baking powder.