Preheat oven to 180˚C.
Grease an 8 inch (20 cm) springform pan and line it with baking paper.
Whisk together spelt flour and baking powder in a bowl and set aside.
In your mixer or a large mixing bowl, beat together eggs and sweetener on high speed for 2 minutes until frothy. (I like to do this with the whisk attachment if using my stand mixer.)
Then add melted butter, ricotta, vanilla paste or extract, lemon zest and beat on medium speed for 1 minute or just until blended. (I like to do this with the flex edge beater attachment if using my stand mixer.)
On a slow speed in the mixer, or manually with a spatula, mix in the flour mixture until well blended.
Pour the mixture into the lined pan and sprinkle the blueberries over the top.
Bake for 45 - 50 minutes until light brown on top and the cake looks set.
If you like, sprinkle a little extra sweetener over the top.