Preheat the oven to 350 F. In a large bowl, whisk together flour, sugar, cocoa powder, salt, and baking soda. Add in melted butter, eggs, and peppermint extract. Whisk until smooth.
Pour half of the brownie batter into a greased 8-inch square baking dish. Evenly place the peppermint patties in a single layer over the batter. Pour the remaining half of the brownie batter over the peppermint patties.