How to Make It
Preheat the oven to 375°. Butter and flour a 12-cup Bundt pan. In a large bowl, using an electric mixer, beat the egg whites with the cream of tartar until soft peaks form. Gradually beat in 1/2 cup of the sugar.
In a medium bowl, whisk the cake flour, baking powder and salt. In a large bowl, using an electric mixer, beat the 6 egg yolks with the water, olive oil, vanilla, lemon zest and the remaining 1 cup of sugar. Add the dry ingredients and beat until the batter is smooth. Using a spatula, fold in the beaten egg whites until no streaks remain.
Spoon the batter into the prepared Bundt pan. Bake the cake for 35 to 40 minutes, until springy to the touch. Let cool for 20 minutes, then invert the cake onto a rack to cool completely. Lower the oven temperature to 350°.
In a small saucepan, simmer the water and sugar over moderate heat just until the sugar is dissolved, about 5 minutes. Let cool, then stir in the limoncello. Brush the syrup all over the cake, allowing it to soak in.
Line a large baking sheet with parchment paper. In a heavy saucepan, stir the sugar with the cream of tartar and water until sandy. Wash down the side of the pan with a moistened pastry brush to remove any sugar crystals. Bring the mixture to a boil over moderately high heat and cook without stirring until a deep honey-colored caramel forms, about 5 minutes. Remove from the heat. Carefully swirl the pan to cool the caramel slightly, then pour it onto the baking sheet in a rough round. Using an offset spatula, spread the caramel into a 13-inch round and let stand until slightly cooled but still pliable, about 5 minutes.
Invert the caramel round over the cake and peel off the parchment paper. Gently press the caramel onto the cake before it hardens to help it conform. If the caramel hardens as you work, place the caramel-coated cake in the oven for 2 to 3 minutes, just to soften the caramel. When the caramel has hardened, serve the cake with the Lime-Yogurt Mousse.