Preheat the oven to 325 degrees F. Grease a 9-inch round spring form pan.
To a stand mixer or a bowl using a handheld mixer, add the butter, ricotta, egg yolks, almond extract, sweetener, stevia and salt. Blend until combined.
Next add the almond flour and lemon zest and blend again until incorporated. Set aside.
Add your egg whites to a clean mixing bowl with cream of tartar. Whip egg whites until stiff peaks form.
Fold half of the egg whites into the almond mixture, then once combined fold in the remaining.
Pour the batter into the pan and sprinkle with slivered almonds.
Bake for 40-45 minutes until the edges are golden and a toothpick in the center comes out clean. Cool 10 minutes then use a butter knife to lossen edges before removing sides of spring form pan. Allow to cool completely then dust with confectioners to serve.
Store in an airtight container in the refirgerator for up to 5 days.