Preheat the oven to 325 degrees F. Line the base of an 8-inch springform tin with parchment paper and set aside.
Place the butter, half of the honey, the lemon zest, and vanilla in a bowl and use an electric mixer to beat everything until creamy. Add the egg yolks and continue
to beat for a further minute. Add the almond flour, polenta and ricotta and fold everything together. Whisk the egg whites in a separate bowl until they reach soft peaks. Add the remaining honey and continue whisking until they reach soft peaks again and it is well blended.
Slowly fold the egg whites into the cake mixture. Turn the mixture into the prepared cake tin and sprinkle the slivered almonds evenly over the top. Bake for 40–50 minutes or until a toothpick inserted in the center comes out clean. The center of the cake might look slightly wobbly at first, but it will firm up when the cake cools down. Leave to cool completely before removing it from the tin.