Hazelnut crunch: In a small bowl, combine hazelnuts, brown sugar and cinnamon. Set aside.
Muffins: In a large bowl, combine buckwheat flour, all-purpose flour, cinnamon, baking powder, baking soda, nutmeg and salt.
In a medium bowl, whisk together brown sugar, eggs, oil and vanilla until blended. Pour over flour mixture and stir until just combined. Fold in peaches.
Divide batter evenly among prepared muffin cups. Sprinkle hazelnut crunch evenly over muffins.
Bake in preheated oven for 15 to 17 minutes or until tops are firm to the touch and a tester inserted in the center of a muffin comes out clean. Let cool in pans on a wire rack for 10 minutes, then transfer to rack to cool completely.