Preheat the oven to 325. Line an 8 x 8 or 9 x 9 square pan with parchment paper.
In a large bowl, whisk together the butter and sugar. Add the eggs and vanilla, and mix until smooth. Add the cake flour, cocoa powder, baking soda, and salt, and mix until just combined. Fold in the Reese's candies.
Spread the brownie mixture in the prepared pan. Bake 40-50 minutes, or until a toothpick inserted 1 1/2 inches from the side of the pan comes out clean or with a few moist crumbs. Let cool before slicing and serving.