Preheat your oven to 350°F (175°C) and grease a 9-inch round cake pan with butter or non-stick spray.

In a large mixing bowl, whisk together all-purpose flour, granulated sugar, unsweetened cocoa powder, melted butter, eggs, vanilla extract, baking powder, and a pinch of salt.

Pour the brownie batter into your prepared cake pan and bake for about 25 minutes or until a toothpick comes out with a few moist crumbs.

Allow the brownie to cool in the pan for about 10 minutes, then transfer it to a wire rack to cool completely.

Spread 4 cups of your favorite slightly softened ice cream evenly over the cooled brownie layer.

Sprinkle any desired optional mix-ins on top of the ice cream layer.

Cover the cake with cling film or aluminum foil and place it in the freezer for at least 4 hours or until firm.

Once frozen, let the cake sit at room temperature for a few minutes before slicing and serving.