Preheat the oven to 175C / 350F / 155C fan assisted. Separate the egg yolks from the eggs whites.
Mix the egg yolks, melted coconut oil (room temperature), lemon zest, juice and optional erythritol together in a mixing bowl.
Blitz the chia or flax seeds in a high speed blender. Add the almond and coconut flours, ground chia or flax, baking soda, coconut yoghurt and salt to the eggs. Mix to combine.
Whip the egg whites, either in a food processor or using an electric whisk. After 30 seconds add the cream of tartar or apple cider vinegar until light and fluffy with stiff peaks.
Fold the egg whites into the dough. Careful not to overwork as you don’t want to flatten the mixture. Fold through the blueberries.
Line a non-stick muffin tray with a little coconut oil or if using muffin cases grease with a little oil to prevent sticking. Alternatively, you can use silicone cases.
Bake in the oven for around 30 - 35 minutes until golden on top and you can insert and remove a skewer without any crumbs sticking.
Allow to cool, approximately 10 minutes, then remove from the tin before serving.
Storage : room temperature for up to 4 days or freeze for 2 months