Preheat the oven to 325. Line a 9 x 9 baking dish with parchment or foil and set aside.
In a large bowl or the bowl of a stand mixer, mix together the butter, brown sugar, and sugar. Add the egg, egg yolk, and vanilla, and mix to combine. Add the flour, baking soda, and salt, and mix until just combined. Using a spatula, gently fold in 3/4 cup of the Reese’s Pieces and the chopped Reese’s miniatures, being careful not to smash the candy.
Spread the dough in the prepared pan and sprinkle with the remaining Reese’s Pieces, pressing them gently into the top of the dough.
Bake 25-30 minutes, or until the edges are lightly browned and the middle looks dry and does not sink in when gently touched. Remove and let cool before serving.