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

Bean 3/4 cup sugar and butter in a large bowl with an electric mixer until creamy, about 3 minutes. Add lemon zest, lemon juice, and vanilla sugar; mix until well combined. Add egg yolks, one at a time, beating well after each addition. Fold in almond flour and ricotta cheese with a silicone spatula.

Wash and dry the beaters. 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 almond-ricotta batter; pour 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 cake cool completely in the pan before transferring to a cake platter, about 1 hour. Dust with confectioners' sugar before serving.