Preheat oven to 350 degrees F.
Line a baking sheet or two with parchment paper.
Using a mixer, cream butter until light and creamy for 2 minutes.
Add in sugar and brown sugar and continuing creaming for 1-2 more minutes.
Add in egg, mixing until fully incorporated into batter.
Add in vanilla extract and mix.
Sift in flour, baking powder and cocoa powder into the mixture, and beat together just until a chocolate dough forms.
Then fold in the Reese's Pieces and white chocolate chips.
Using a medium cookie scoop, (1 & 1/2 tablespoons), scoop cookie dough onto baking sheet and press some Reese's Pieces on top, but don't add more white chocolate chips until they are done baking. This helps the white chocolate chips not to brown on top.
Leave room for the cookies to spread very slightly.
Bake for approximately 8-10 minutes. Gooier for 8 minutes.
Cookies should be starting to firm up but still soft in the middle. Remove from the oven and press a few white chocolate chips if desired.
Let cookies set on cookie tray for a couple of minutes before removing them to cool completely on a wire rack. So good with a glass of milk!
Allow to cool completely prior to storing.