Mirror, mirror on the wall, who is the shiniest cake of all? Thanks to gelatin, it’s this one!
Line rimmed baking sheet with parchment paper and place cooling rack inside baking sheet. Place frosted cake on 9-inch cardboard round and place in freezer while making glaze.
Pour ½ cup glaze into each liquid measuring cup. Add gel food coloring to each glaze and combine in bowl following photos, below.
Let cake sit for 5 minutes, allowing extra glaze to drip off cake. Slide spatula under cardboard round and carefully lift glazed cake. Transfer glazed cake to serving platter and place in refrigerator to chill until glaze is set and no longer sticky, about 20 minutes.
The shiny, reflective surface of a mirror cake is all thanks to the magic of gelatin. When the gelatin in the Mirror Cake glaze is above 50 degrees, it’s a pourable liquid. But when gelatin’s temperature drops below 50 degrees, it begins to turn solid. And the colder gelatin gets, the faster it solidifies—that’s why we freeze the frosted cake before pouring the glaze on top. When the warm glaze hits the cold cake, its temperature drops and the gelatin starts to become a solid. As it cools, it traps the water inside a tangled mesh of gelatin molecules. After the glaze sets in the refrigerator for 20 minutes and its temperature reaches below 50 degrees, the water trapped inside this solid gel acts like a smooth, reflective pond, puddle, or mirror!
Lining cake pans with parchment paper rounds prevents cakes from sticking and makes them easier to remove. Cardboard rounds help with moving frosted cakes from one place to another. These pictures show parchment paper, but the technique is the same for cardboard rounds.