How to Make It
Butter a 10-inch springform pan. Line the bottom with parchment paper and butter the paper.
In a large bowl set over a pot of barely simmering water, stir the chopped white chocolate with the butter until almost melted. Remove the bowl from the heat, add the vanilla seeds and stir until smooth. Stir in the egg yolks. Sift the bread flour over the batter and stir it in.
In a large bowl, beat the egg whites with the cream of tartar to soft peaks. Gradually add the sugar and beat until the whites are stiff and glossy. Beat one-fourth of the egg whites into the batter, then fold in the remaining whites until no streaks remain. Pour the batter into the prepared pan and smooth the top. Cover with plastic wrap and refrigerate for 8 hours or overnight.
Preheat the oven to 375°. Bake the cake in the middle of the oven for about 30 minutes, until puffed and golden and a toothpick inserted in the center comes out clean. Let the cake cool completely in the pan; it will fall dramatically.
Run a thin knife around the edge of the cake, then remove the side of the pan. Slide the cake onto a plate. Using a vegetable peeler, shave white chocolate curls over the cake.