Preheat oven to 350 degrees F (175 degrees C) and grease a 9x13-inch baking dish.
Combine cake mix, water, eggs, and oil in a large bowl; beat with an electric mixer on medium until smooth, about 2 minutes. Pour batter into prepared dish.
Bake 26 to 30 minutes until a toothpick inserted into the center comes out clean. Allow to cool completely on a wire rack.
Ice cream
Remove carton paper from ice cream; set the block on its long side and use a piece of string or dental floss to cut it in half lengthwise. Place the two pieces side-by-side on a sheet of waxed paper.
Remove cooled cake from the baking dish and place it over the ice cream slabs. Trim cake and ice cream so the edges are flush.
Place a board or serving platter over the cake, hold the board and waxed paper, and flip the ice cream cake over. Remove the waxed paper and smooth the seam between the ice cream slabs.
Cover the top with waxed paper and freeze until very firm. Decorate as desired before serving.
Tips
Let the baked cake cool completely before trimming and assembling.
Bake until a toothpick comes out clean; avoid overbaking to keep the cake moist.
Let the frozen cake sit at room temperature about 30 minutes before serving.