You can either bake your cake in two small round cake pans OR in a 9x13 and cut it in three even rectangles.
Once your cake is cooled, soften your ice cream for 10-15 minutes at room temp, or briefly in the microwave.
FOR A ROUND CAKE: Lay down your first layer of chocolate cake on a serving plate or cake stand and top with half of the ice cream. Smooth with a spoon, trying to keep the layer even and smooth. Top with second layer of chocolate cake and press down gently. Top with other half of ice cream and "frost" the cake with it. You can either frost it all the way around or just on the top. It doesn't have to be perfect. Cover and freeze for at least 1 hour. Before slicing leave out for 20-30 minutes to soften.
FOR A RECTANGLE CAKE: Line a loaf pan with parchment paper and lay down your first layer of chocolate cake. Then top with half of your mint ice cream and smooth with a spoon. Repeat twice, ending with cake on the top layer (which will actually be the bottom once inverted).
Cover and freeze for at least 1 hour. Before slicing leave out for 20-30 minutes to soften.
I had some leftover raw brownie from the ice cream recipe, which I used to top the cake. This is optional.