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Preheat the oven to 350 F. Butter a 9″ springform pan (or other cake pan), and line with buttered parchment paper.
With a mixer, cream the butter and 1 cup sugar together until light and fluffy. Add the vanilla and lemon zest, beat again until fully combined.
Add the egg yolks to the mixture, one at a time, mixing fully between each.
Mix in the ricotta, until smooth.
Add the almond meal, and mix until fully combined.
In a separate bowl, whip the eggwhites until soft peaks form. Slowly add the remaining 1/3 cup sugar and whip until stiff peaks form. In three batches, fold the egg whites into the almond meal batter. Continue folding until completely smooth and incorporated.
Pour the mixture into the prepared pan, smooth the top, and bake for 45-50 minutes, or until not longer jiggly in the middle. Remove, and let cool completely before topping with flaked almonds and dusting with powdered sugar.