Preheat oven to 180 degrees C (350 degrees F). Line a 17-18 cm (7 inch) cake pan with a circle of parchment paper. Grease well the pan on top.
In a medium bowl combine all purpose flour, coconut flour, baking powder and salt. Mix with a spoon to remove any crumbs.
In a large mixing bowl with an electric mixer beat together eggs and sugar until the mixture triples its volume. Add butter and vanilla and beat until absorbed. Add yogurt and whisk shortly, until combined.
Gradually add the dry ingredients, mixing gently with wooden spoon until absorbed.
Pour the batter into the prepared pan. Bake for 40-50 minutes or until a wooden skewer inserted in the centre comes out clean.