Super chocolatey with a deliciously gooey center, these Double Chocolate Brownie Cupcakes will be your new favorite dessert!
Preheat oven to 325 degrees. Generously spray a muffin tin with nonstick cooking spray or line with cupcake liners.
In a large bowl, combine melted butter, sugar, cocoa powder and vanilla until well blended. Add eggs one at a time, stirring well after each addition. Add the flour and stir until well blended. Stir in the walnuts.
Spoon batter evenly into muffin cups filling just a little over 3/4 full. Sprinkle each cupcake wirh some semisweet chocolate chips or chopped semisweet chocolate pieces.
Bake from 18-20 minutes. Edges should be set but the cupcakes should still be gooey in the middle. Cool in pan for 10 minutes, then remove carefully. Repeat with any remaining batter. Serve immediately with ice cream on the side, if desired.