I’ve made a couple of ice cream cakes over the years, and my method is improving. But I think I’ve got a pretty solid method (and ice cream ratio) figured out now! So here it is.
1. Bake cakes ahead of time. Use a large knife to level the heights, then double-wrap in plastic wrap, place in a plastic ziploc bag, and freeze. Keep flat in the freezer.
2. Prep the same cake pan for the ice cream layer. Line the cake pan with saran wrap. Cut the cake scraps from step 1 into strips the height of each cake layer, and line the cake pan around the outside. Crumb side out works best for a smooth outside layer.
3. Use an ice cream scoop and/or spoon to scoop small amounts of ice cream into the cake pan. Press each bit of ice cream down with gloved fingers to smooth out the layer. Repeat until the layer is fully solid, and flat-ish. Cover the entire layer with a layer of saran wrap, then use fingers to press down to fill any gaps. Add more ice cream in areas where needed. Place the layer in the freezer for a few hours.
4. Assembly the layers of cake, ice cream, then cake. Frost as you would with a normal cake, but popping the cake back into the freezer every 5-10 minutes for a few minutes. I typically use a whipped cream frosting or ganache on an ice cream cake, since they freeze well.
It looks like a giant ice cream sandwich, with vanilla cookies or something.