To better synchronize the downtime in both recipes, start the Marshmallow Buttercream before the cake. First, make the marshmallow base: In a small bowl, mix the gelatin with 2 ounces (1⁄4 cup) cool tap water and vanilla extract, if using. If using a vanilla bean, split lengthwise with a paring knife, run the flat of the blade down each half to scrape out the seeds, and add to the gelatin without stirring. (Reserve the pod for another project.)
Combine remaining 6 ounces (3⁄4 cup) water, corn syrup, sugar, and salt in a 3-quart stainless steel pot and set over medium heat. Stir mixture with a fork until bubbling, about 5 minutes, then increase heat to medium-high. Clip on a digital thermometer and cook, without stirring, until the clear syrup registers 250°F, about 8 minutes.
Transfer thermometer to the bowl of a stand mixer and pour in the hot syrup all at once, scraping the pot with a heat-resistant spatula. Cool to exactly 212°F, about 8 minutes, then add gelatin. With the whisk attachment, mix on low speed until the gelatin is melted, then increase to medium-high and whip until thick, snowy white, roughly tripled in volume, and beginning to ball up around the whisk, about 10 minutes. Scrape into a greased 4-cup container, cover tightly, and let stand at cool room temperature until thick and firm, at least 2 hours, or up to 1 week.
While the marshmallow cream is resting, start the cake: Adjust an oven rack to lower-middle position and preheat to 325°F. Line three 8-by-3-inch anodized aluminum cake pans with parchment and grease with pan spray; if you don’t have three pans, the remaining batter can be held at room temperature for up to 3 hours. (The cakes will brown more and rise less in 2-inch pans.) Sift the flour (if using cup measures, spoon into the cups and level with a knife before sifting) and set aside. Combine butter, coconut oil, sugar, baking powder, baking soda, and salt in the bowl of a stand mixer fitted with a paddle attachment. Mix on low speed to moisten, then increase to medium and cream until fluffy and light, about 5 minutes, pausing to scrape the bowl and beater halfway through. With the mixer running, add the egg whites one at a time, followed by vanilla and almond extracts.Reduce speed to low and sprinkle in one-third of the flour, followed by one-third of the buttermilk. Alternate between the two, allowing each addition to be roughly incorporated before adding the next. Once smooth, fold with a flexible spatula to ensure it’s well mixed from the bottom up. Divide among the prepared cake pans, about 22 ounces each.
Bake until the cakes are firm but pale, browned only around the very edges, about 40 minutes (or 210°F). A toothpick inserted into the center will emerge with a few crumbs still attached, and your fingertip will leave a slight indentation in the puffy crust. Cool until no trace of warmth remains, about 90 minutes.
While the cakes are cooling, finish the buttercream. Transfer marshmallow base to the bowl of a stand mixer fitted with the whisk attachment. Whipping on medium speed, begin adding the butter 1 tablespoon at a time, waiting for about 5 seconds after each addition. The fluffy creme will cling to the whisk at first but loosen as the butter is incorporated. Once combined, scrape the bowl with a flexible spatula and whip a minute more. The buttercream should be light and creamy but thick enough to hang upside down from a spoon. If it seems stiff or dense (feeling greasy rather than melting on your tongue), scoop a cup into a small bowl and microwave until completely melted, about 30 seconds. Return the melted buttercream to the bowl and whip 15 seconds on medium-high. Conversely, if it seems loose or gooey, refrigerate the entire bowl 15 minutes, then whip 3 minutes on medium-high.
Loosen the cakes from their pans with a knife. Invert onto a wire rack, peel off the parchment, and reinvert. Crumb-coat and frost with Marshmallow Buttercream. Serve immediately or store under a cake dome or an inverted pot; the frosted cake will keep for up to 24 hours at room temperature. After cutting, wrap leftover slices individually and store at room temperature for up to 2 days more.