Line a 9- x 9-inch baking pan with parchment paper or wax paper, allowing paper to overhang the sides (this eases removal of the slab from the pan).
Make the slab: In a large pot, melt the butter over medium-low heat.
When butter is melted, stir in the marshmallows and ⅛ teaspoon salt (if using unsalted butter). Melt and cook, stirring often, until mixture turns pale brown, then stir constantly until lightly browned but not dark, 3 to 5 minutes.
Remove from heat, then add cereal and mix well, preferably with a silicone spoon or a spatula. Scrape the cereal mixture into the prepared pan and press down firmly. If necessary, butter your hands to press mixture flat or use a small piece of plastic wrap.
Let the cereal mixture cool, then remove the slab from the pan.
Assemble the cake: Make as many batches as needed for the sculpted cake. Stack slabs of crisped rice treats, using frosting to hold the layers together. Carve the stack as needed for the sculpted cake design.
Recipe Notes
To make softer, more traditional, Crisped Rice Treats, use ½ cup butter (113 grams) and ½ box crisped rice cereal (170 grams). After cooling the treats, cut the slab into squares or bars as desired.For an even more homemade treat, consider making your own marshmallows. You can flavor them as you'd like (maple marshmallows? whiskey marshmallows?). Homemade marshmallows are easy and fun!