This deliciously sweet, sour, and cakey trifle came about after tinkering with a mess of vinegar-macerated kumquats and then roasting them. I tossed them in with Alice Medrich's White Chocolate Whipped Cream and Julia Child's butter sponge cake, from Mastering the Art of French Cooking, Volume One (1961). I love this cake. It proves that folding egg whites can be meditation, and makes me feel like I can tackle any French dessert. It's delightfully light, yet moist, and makes the perfect base for the trifle. However, you could use just about any vanilla-forward cake, like angel food. —Samantha Weiss Hills
4 to 6, depending on serving size (you may have some cake leftover)
Champagne vinegar, more or less to taste
plus 4 tablespoons granulated sugar, divided
Macerate the sliced kumquats in the Champagne vinegar and 2 tablespoons of sugar. Feel free to layer in (or leave out) the ground ginger. This mixture can sit for just 30 minutes or up to 4 hours before you are ready to serve, depending on your time constraints or flavor preferences. Kumquats don't expel as much juice as their other peeled cousins, so once everything is in a bowl together, I'd give it a good mix and mash with my hands.
Since it needs to chill for a few hours before you whip it, this is a good point to make the white chocolate and heavy cream mixture for the whipped cream, using Alice Medrich's recipe: https://food52.com/recipes/26463-white-chocolate-whipped-cream. Store it in the refrigerator after it has cooled to await whipping.
Line a 10-inch cake pan with buttered parchment paper. Measure out all of your cake ingredients (including the remaining sugar).
Melt the butter and set aside to cool.
Gradually beat 2/3 cup of sugar into the egg yolks, add the vanilla, and continue beating for several minutes until the mixture is thick, pale yellow, and forms ribbons when you drizzle it.
Beat the egg whites and salt together in a separate bowl until soft peaks are formed; sprinkle on the remaining 2 tablespoons of the sugar and beat until stiff peaks are formed. Scoop 1/4 of the egg whites over the top of the egg yolks and sugar mixture. Sift 1/4 of the cake flour on top, then delicately fold in until partially blended. Then add 1/3 of the remaining egg whites, sift in 1/3 of the remaining flour, fold until partially blended, and repeat. Then add the last of each and half of the tepid, melted butter. When partially blended, fold in the rest of the butter but omit the milky residue at the bottom of the pan. Do not overmix; the egg whites must retain as much volume as possible.
Turn the batter into the prepared cake pan. Spread the kumquats onto a baking sheet. Set the cake in the middle level of your preheated oven for 30 to 35 minutes, along with the macerated kumquats. The cake is done when it has puffed, is lightly brown, and has just begun to show a faint line of shrinkage from the edge of the pan. (I think this last indicator is really key; it's a helpful visual cue to look for.) The kumquats can come out at the same time; you want them to have the consistency of a chunky compote or spread—they won't be as syrupy or liquidy as some trifle fillings.
While the cake and kumquats are in the oven, whip your premade heavy cream and white chocolate mixture to preferred consistency.
Remove the cake and the kumquats from oven and let stand in the pan for 6 to 8 minutes. The cake will sink slightly and shrink more from the edges of the pan. Run a knife around the edge of the pan, and reverse on cake rack (or plate, in my case), giving the pan a sharp little jerk to dislodge the cake. Allow to cool for an hour or so. The kumquats can be served room temperature or chilled.
Assemble your individual trifles in a casual way: While you could cut the cake into uniform cubes, I'd tear it into a mismatched group. Toss about 4 or 5 cubes into each dessert bowl, top with a healthy dollop (or two) of white chocolate whipped cream, and as little or as much roasted kumquats as you'd like.