Preheat the oven to 160°C (320°F). Grease a 7-inch cake pan with butter and line the base with parchment paper.

Add the butter, sweetener, and vanilla extract to a large mixing bowl. Beat until creamy and well combined.

Add the eggs one at a time, beating well after each addition until light and frothy.

Add the ricotta cheese, lemon zest, and lemon juice. Whisk until fully combined.

In a separate bowl, combine the almond flour and baking powder, breaking up any lumps.

Add the dry ingredients to the wet ingredients in two batches, mixing after each addition until combined.

Pour the batter into the prepared cake pan and spread evenly.

Bake for about 50 minutes, until the cake is cooked through.

Allow the cake to cool before removing it from the pan.

Slice and serve.