Preheat oven to 350F (175C). Grease a 9-inch round or 8x8 baking pan.
Whisk together almond flour, coconut flour, erythritol, baking powder, and salt.
In a separate bowl, beat eggs, yogurt, butter, lemon zest, and vanilla. Add wet to dry and mix until smooth.
Pour into pan. Bake 30-35 minutes until a toothpick comes out clean.
While hot, poke holes across the top with a skewer.
Heat lemon juice, erythritol, and water until erythritol dissolves. Pour slowly over hot cake.
Cool 20 minutes before slicing.