Preheat oven to 350 F and lightly grease a 12-cup muffin tin (or line with papers).

Melt butter in a pot and leave to cool. Mix with eggs and yogurt in a bowl.

In a separate bowl, beat flour, baking powder and sugar. Add this combination to the egg mixture and gently whisk by adding the flour from the side to the center of the container.

Add the hips and the lemon zest. Scoop the batter into the moulds and sprinkle with a tiny bit of sugar and a handful of oat flakes.

Place in the oven and bake for 25 to 30 minutes or until the muffins are well puffed and golden brown. Let them cool for 5 minutes, then remove them to a wire rack to cool completely. The muffins can be kept in a hermetic box for three days, or in the freezer.