18 oz. (4 1/2 sticks) unsalted butter, cold and cubed
1 tbsp. vanilla extract
Orange food gel coloring
Preheat your oven to 350F/175C, and grease and line three 6″ cake pans.
In a bowl, melt your butter, whisk in the milk, and warm until warm to the touch. Set aside to cool, stirring occasionally so the butter and milk don’t separated. In the bowl of a stand mixer, beat your eggs on high until light and fluffy. Add the granulated sugar and beat until dissolved. Using a spatula, gently fold in the flour, baking powder, and vanilla extract until just combined. Fold in the milk-butter mixture, and divide the batter evenly between your three pans. Bake for 20-25 minutes, or until a toothpick inserted in the center comes out clean. Turn the cakes out onto a wire rack to cool for about 5 minutes. Once the cakes are cool enough to handle (but they should still be warm) wrap them in plastic wrap and set aside in the refrigerator to cool completely. Wrapping them while they’re still warm will help lock in the moisture.
Wash and dice your ripe peaches, and puree in a blender. Pour half of your puree into a small saucepan, and stir in your sugar and gelatin. Cook the mixture over medium heat until hot, but not boiling. Remove the mixture from the burner, and stir in your remaining peach puree. Cover the peach mixture with plastic wrap pressed down against the surface of the mixture, and set aside to cool in the refrigerator.
Because we’re going to need to assemble everything at once, now is a good time to whip up your frosting (we’ll finish up the peach mousse in a bit). Using a double boiler with the bowl of your stand mixer, whisk together your egg whites and sugar, and heat until the mixture is hot and the sugar dissolves. Remove the bowl from the double boiler, and beat on high using the whisk attachment until stiff peaks form and the mixture has cooled to room temperature (8-10 minutes). Switch to your paddle attachment, and begin adding your cubes of cold butter while mixing on low-medium. Once all the butter is in, increase the speed to high and beat until a smooth frosting forms. Add the vanilla extract and mix until just combined.
Now that the peach mixture has cooled, it’s time to finish the mousse. Plunge the bowl of peach into an ice bath, and stir as the mixture begins to thicken. Once the mixture thickens to the consistency of pudding, whip your heavy cream, and gently fold it into the peaches until just combined.
To assemble your cake, start by spreading a thin layer of frosting on the first layer. Spoon a small amount of your frosting into a bag, and pipe a rim around the edge of your frosted layer. This will act as a dam to keep the mousse from being pushed out by the weight of the cake. Fill the center with mousse, and top with your second layer. Repeat this process for the second layer. Once your cake is filled, give it a light crumb coating with the frosting, making sure the areas between the layers are completely filled in, and allow to chill at least 3 hours or up to overnight so the mousse can set. Cover your leftover frosting with plastic wrap so it doesn’t crust.
Once the cake has chilled sufficiently, you can finish frosting the outside. Begin with the top of the cake, making it as even and smooth as possible. Divide the remaining frosting between 3 bowls, leaving one uncolored, and coloring the other two different shades of orange (or peach, if you will). Spoon your frosting into piping bags, and starting at the bottom with the darkest shade, begin piping your frosting on and around your cake. Once you have all your frosting on the cake (be generous with your frosting as it’s easier to take it off later than put more on), use an offset spatula to smooth it out. If you want an extra smooth finish on your frosting, you can wait 30 minutes to an hour for the frosting to begin to crust, and then go over it one more time with your offset spatula.