makes one 6-cup bundt (or 6- or 8-inch round or square); adapted from Dorie Greenspan's Quatre-Quarts
Preheat oven to 375°F. Butter a 6-cup Lotus bundt (or 6- or 8-inch round or square) and dust with flour (or use baking spray). Melt butter and set aside to cool.
Whisk together flour, baking powder, and salt; set aside. Using an electric mixer with whisk attachment, beat egg whites until foamy, sprinkle in cream of tartar, and continue to beat until they hold firm peaks. Transfer to another bowl and set aside.
Using whisk or paddle attachment, beat egg yolks, sugar, and cooled, melted butter until thick and pale yellow. If using compote, mix it with the strawberries and rhubarb. Add the fruit and vanilla to the batter and mix. Sprinkle in flour and mix just until combined. Fold one quarter of the egg whites into the batter. Gently fold in the remainder, scoop batter into prepared pan, and smooth the surface.
Bake for 40–45 minutes (slightly less for a layer cake pan, 5 minutes), until top is golden brown. Let cool 10 minutes in pan then, if desired, invert onto a cooling rack. Keep loosely covered to preserve the crust. Enjoy plain or with strawberry rhubarb compote.