Preheat the oven to 170°C.
Melt the white chocolate in a microwave or double boiler.
In a separate bowl, melt the butter in the microwave.
In a third bowl, beat the egg yolks and sugar until light and foamy. Mix in the melted butter and white chocolate, then add the zest of one lemon and the juice of half a lemon.
Stir in the almond flour, cornstarch, and baking powder until the mixture is thick but smooth.
Beat the egg whites until stiff peaks form. Fold one-third of the egg whites into the batter vigorously to loosen it, then gently fold in the remaining whites.
Pour the batter into a buttered mold lined with parchment paper at the bottom.
Bake for about 35 minutes, or until the cake is golden and a knife inserted comes out clean. Allow to cool for 10 minutes before unmolding, then let cool completely. Dust with powdered sugar before serving.