For the chestnut-apple muffins, beat the eggs, sugar, butter and yogurt until foamy. Peel the apples, grate coarsely and mix into the batter together with the cinnamon.
Coarsely chop the chestnuts. Reserve about 2 tablespoons of chestnuts for sprinkling on the muffins. Stir the remaining chestnuts into the muffin batter and then finally stir in the flour sifted with the baking powder.
Pour the batter into the muffin cups and sprinkle the muffins with the remaining chopped chestnuts and some brown sugar. Bake the chestnut-apple muffins in the preheated oven at 180°C for about 25 minutes.