We go rainbow crazy every June when Pride Week is celebrated in Boston and around the country. Every color of the rainbow is baked into cupcakes, folded into Oreo filling, even mixed through rice crispy treats. For over a week it looks like a carnival let loose on the pastry counter at Flour. I don't know what it is about rainbow sprinkles, but put them on a cake or in a cookie and you have an instant hit. This Funfetti angel food cake covered in toasted meringue is one of the prettiest--and for me tastiest--rainbow specials we offer. The meringue helps keep the cake tender by protecting it from drying out, but I doubt you'll have a lot of leftovers. We never do!
For the rainbow confetti angel food cake: Preheat the oven to 350 degrees F and place a rack in the center of the oven.
Place the egg whites, vanilla, 1 teaspoon water, and the cream of tartar in a stand mixer fitted with a whisk attachment (or in a large bowl, using a whisk). Whip on medium speed for 2 to 3 minutes (or 5 to 6 minutes by hand) until you can see the wires of the whisk leaving a slight trail in the whites.
With the mixer still on medium, or while continuing to whip vigorously by hand, slowly add 3/4 cup (150 grams) of the sugar, a few tablespoons at a time. After adding a little sugar, let it whip into the whites for 10 seconds or so, then add more sugar, then again wait about 10 seconds. Continue this way until all the sugar is added; that should take at least 1 minute. Once all the sugar is added to the whites, whip on medium-high for 2 to 3 minutes more (5 to 6 minutes by hand), until the whites are glossy and shiny and hold a peak when you slowly lift the whisk straight up and out of them.
Stir together the cake flour, the remaining 3/4 cup (150 grams) sugar, the sprinkles, and salt in a small bowl. Carefully sprinkle the flour mixture on top of the whites and gently and quickly fold it in. When the everything is entirely mixed in, scrape the batter into an ungreased 9-inch angel food cake pan. Use a spoon or offset spatula to even off the top of the batter.
Bake for 30 to 40 minutes, rotating the cake pan midway through the baking time, until the entire top of the cake is light golden brown and the cake springs back when you press it lightly, and a cake tester inserted into the middle of the cake comes out clean. Immediately remove the cake from the oven and turn the pan upside down. (This ensures that the cake does not deflate and compact onto itself; since the cake pan is not greased, the cake won't fall out.) Leave the cake upside down until it is entirely cooled, at least 2 hours.
When the cake is cool, make the frosting.
Turn the cake pan right side up. Remove the cake from the pan by first running a knife around the outer edge of the cake and then around the inner tube of the pan; pop out the removable bottom along with the cake, then run a knife along the bottom of the cake to separate it from the pan. Turn the inner tube and pan bottom upside down and remove the cake carefully and quickly, then turn the cake right side up again onto a serving plate. Frost the sides, top, and inner tube with the frosting. Use an offset spatula or knife to smooth the frosting. Use a blowtorch to carefully torch the frosting all over to create a golden brown surface. If you do not have a torch, place the frosted cake on a baking sheet under the broiler and rotate it every few minutes until the sides and top are toasted, 8 to 12 minutes total. Serve immediately, or store at room temperature and serve within 6 hours at the very longest. (The frosting will not last for more than 6 hours at room temperature, and the cake itself does not store well in the fridge.)
For the fluffy marshmallow frosting: In a small metal or heatproof glass bowl, whisk together the sugar and egg whites to make a thick slurry. Place the bowl over a small pot of simmering water (make sure the pot is small enough that the bowl sits above the water and not directly touching it, or you may end up with scrambled egg whites) and cook, whisking occasionally, until the mixture is very hot to the touch, 6 to 8 minutes. (When you stick your finger in the mixture it should be so hot you immediately remove your finger.) It will thin out a bit as the sugar melts.
Scrape the sugar-egg white mixture into a stand mixer fitted with a whisk attachment and whip on medium-high speed for 6 to 8 minutes, until the frosting cools, is fluffy and glossy, and looks like shaving cream. Add the vanilla and salt and whip until mixed in. Use immediately.
Propane gas torches are highly flammable and should be kept away from heat, open flame and prolonged exposure to sunlight. They should be used only in well-ventilated areas. Follow torch manufacturer's instructions for use.