Heat the oven to 170 C / 338 F. Grease a 22 cm / 9-inch springform pan and line the base with parchment paper.
Melt the white chocolate in short 20-second bursts in the microwave, or over a pan of barely simmering water, stirring until smooth. Set aside to cool slightly.
In a large bowl, beat the softened butter with 120g sugar until pale and fluffy, about 3 minutes. Add the egg yolks one at a time, beating well after each.
Beat in the melted white chocolate, lemon zest, and lemon juice until combined. Fold in the almond flour, baking powder, and a pinch of salt with a spatula until no dry streaks remain.
In a clean bowl, whip the egg whites with a pinch of salt until foamy. Gradually add the remaining 80g sugar and whip to soft, glossy peaks that gently droop, not stiff peaks.
Fold a third of the whipped whites into the almond batter to loosen it. Fold in the rest in two more additions, keeping as much air in the batter as you can.
Pour the batter into the prepared pan and smooth the top with the spatula.
Bake for 40 to 42 minutes, until the top is cracked and golden and a skewer inserted 5 cm from the center comes out with a few moist crumbs, not wet batter.
Cool the cake in the pan on a wire rack for 20 minutes, then release the springform sides and let it cool completely.
Dust with powdered sugar just before serving.