Preheat the oven to 325 degrees F (165 degrees C). Grease an 8-inch springform pan and line with parchment paper.

Combine butter and 3/4 cup sugar in a large bowl; beat with an electric mixer until creamy, about 3 minutes. Add vanilla sugar, lemon zest, and lemon juice; mix until well combined. Add egg yolks, 1 at a time, beating well after each addition.

Fold in almond flour and ricotta cheese with a silicone spatula.

Wash and dry the beaters and beat egg whites in a glass, metal, or ceramic bowl until foamy. Gradually add 1/3 cup sugar, continuing to beat until stiff peaks form. Fold stiff egg whites into the batter. Pour batter into the prepared pan. Sprinkle with sliced almonds.

Bake on the center rake of the preheated oven until edges of the cake are lightly browned, about 50 minutes. Cake will still be a little wobbly in the middle.

Remove from the oven and let the cake to cool completely in the pan before transferring to a cake platter, about 1 hour. Dust with confectioners' sugar before serving.