Preheat your oven to 175°C (350°F) to ensure even baking.
Line your loaf pan with parchment paper and grease it lightly with coconut oil.
Separate the egg whites from the yolks, placing the yolks in one bowl and the whites in another.
In the egg yolk bowl, whisk together melted butter, yogurt, lemon juice, lemon zest, coconut butter, coconut oil, and vanilla extract.
In a separate bowl, mix almond flour, coconut flour, baking powder, sweetener, and salt until there are no clumps.
Combine the wet mixture with the dry ingredients and stir until just combined.
Beat the egg whites until stiff peaks form, then gently fold them into the batter.
Pour the batter into the prepared loaf pan and smooth the top. Bake for 45-55 minutes.
Allow the cake to cool in the pan for 10 minutes before transferring it to a wire rack to cool completely.