First, preheat the oven to 180°C or 350°F . (I baked it using the bottom-top and air circulation modes.)
Cut the butter into cubes and melt it in a saucepan. It should not be hot, just lukewarm.
Take out a large mixing bowl. Pour in the xylitol. Add the melted butter and mix with a whisk until fluffy.
Add the whole eggs. Continue whisking with a whisk.
Add the flour, then sprinkle in the baking powder. Pour in the plain yogurt. Stir with a wooden spoon. It should be creamy.
Take out the muffin pan. (I use a silicone mold and place it in a metal pan.) Use a spoon to fill the silicone molds with the batter. Fill 12 molds.
Place the filled molds in the oven and bake at 180°C or 350°F for 30 minutes.
When ready, remove from the oven and leave to cool completely.