In a large mixing bowl, using an electric mixer, cream together the butter, erythritol and one egg until light and fluffy. Add the ricotta, vanilla extract and lemon zest to the bowl, and mix until all ingredients are well combined.
In a separate mixing bowl, combine the almond flour and baking powder and whisk well to combine.
Alternate adding a little almond flour, and one egg at a time to the batter. Once all ingredients are added, mix until all ingredients are well incorporated.