Start by preheating the oven to 325°F (165°C). Grease 6x6 or 7x7 baking pan with butter, then line it with parchment paper, cutting it to fit.
Place the butter, sweetener, and vanilla in a large bowl and cream them together until well blended.
Add one egg and beat until fluffy. Add the ricotta cheese, lemon juice, and lemon zest, then beat until well combined.
Mix one third of the dry ingredients into the batter. Add an egg and mix again. Repeat this process two more times with the remaining dry ingredients and eggs – one third at a time.
Spread the batter into the prepared cake pan with a spatula, and bake for 45-50 minutes. You can insert a toothpick in the middle of the cake to check if it’s done baking – it should come out clean.
Remove the cake from the oven and let it cool for 20 minutes. Then, run a knife around the pan to loosen the cake and flip it onto a rack to continue cooling. Once cooled, you can serve and enjoy. If desired, you can dust the top with sugar-free powdered sweetener.