Preheat the oven to 350 F. Line a 12 cup muffin pan with paper liners. In a large bowl, whisk together flour, sugar, cocoa powder, salt, and baking soda.
Add in eggs, melted butter, and vanilla extract, whisk until smooth. Fold in the Reese's pieces. Pour the brownie batter into the prepared muffin pan. Bake for 15-20 minutes.