Preheat oven to 350F and grease a large springform pan.

In a clean medium sized bowl, beat the egg whites on high with a hand mixer until peaks form. Slowly add ¼ cup of the sugar while beating until stiff peaks form. Set aside.

Cut a slit in the vanilla bean length-ways, open it up, and scrape out the seeds with the back side of a knife. If you don't have vanilla beans you can use vanilla extract.

In a large bowl, beat the butter and vanilla seeds until light and fluffy, about 3 minutes.

Add the remaining ¾ cup sugar, egg yolks, ricotta cheese, lemon zest and lemon juice, and beat on medium until smooth. Add the ground almonds, flour, baking powder and salt and beat again on medium speed until combined.

Gently fold in the egg whites just until the batter is smooth.

Spread the batter into the prepared pan and even it out with the back of a spatula. Sprinkle the cake evenly with the sliced almonds and bake for 45 minutes until the cake is lightly browned on the edges and the edges start to pull away from the sides of the pan.

Allow the cake to cool for 20 minutes before removing the sides of the springform pan. Sprinkle the cake with powdered sugar if you wish and serve.