In a large glass bowl, mix the almond flour, coconut flour, xanthan gum, baking powder, erythritol, and salt together until each of the ingredients are indistinguishable.
This process has to be done quickly, otherwise, it will not work! - Cut the cold butter up into small chunks, and add it to the dry ingredients. Press the butter into the dry ingredients by using two forks. This can also be done in a food processor by pulsing the chunks of butter and dry ingredients for 20 - 30 seconds on high
In a 22cm (8.5") springform cake tin, line the bottom insert with baking paper, grease the inside with butter and press the base mixture into the bottom of the tin. I usually press it halfway up the sides as well.
Place the tin in the fridge to allow the base to set. Preheat your oven to 140 degrees C (280 F) if fan forced, otherwise 160 degrees (320 F).
In a mixing bowl, add the cream cheese, sour cream, vanilla extract, lemon zest, and erythritol. Mix until well combined.
As the mixture begins to thicken slightly, add the first egg and continue to mix. Add in the rest of the eggs one at a time, and continue to mix.
Take the base out of the fridge, poke holes in the bottom of the base with a fork, and bake in the oven for 15 minutes until slightly golden. Remove and let cool slightly. Wrap the bottom of the tin in aluminum foil.
Using a sieve, pour the cream cheese filling through into the base (this helps remove any large clumps that might spoil the cheesecake). Put the cheesecake into the oven and bake in a large baking sheet filled with water (otherwise known as a water batch). Alternatively, you can place 2 ramekins filled with water right next to the cheesecake. Bake for 50 minutes.
Check the cheesecake by pushing a spike into the middle. If it comes out clean then the cake is ready.
Turn the oven off, and leave the oven slight open for another 30 mins to cool the cheesecake slowly. (this stops the top from cracking) Remove from the oven and let it come to room temperature before placing in the fridge for 4 hours to set. Keeps up to 5 days in an airtight container in the fridge.
** Nutrition per serving: Calories 384, Fat 5g, Protein 9g, Carbs 7g