1. Preheat the oven to 420°F (215°C). Lightly grease a muffin tin and set it aside. Melt the butter and let cool down a bit.

2. Mix the two flours, sugar (or xylitol), baking powder, salt, nutmeg, and the blueberries in a large bowl. Set aside.

3. Add the egg and egg yolk to a second bowl and whisk lightly. Add the yogurt, melted butter, lemon zest, lemon juice, and vanilla extract and mix well to combine. Add the flour mixture to the bowl and fold in – do not overmix and try not to mash the blueberries. Divide the batter between the muffin cups, press several blueberries into the dough (optional) and sprinkle with some coarse sugar (also optional). Bake for about 25-30 minutes or until a wooden skewer inserted into the center of a muffin comes out clean. Take out of the oven, let cool down for a moment inside the tin, then remove the muffins and let cool down completely on a wire rack.