Place a rack in the upper third of the oven and preheat to 350 degrees F. Butter and flour a 12-cup muffin pan and set aside. You can also use cupcake papers for this recipe.

To make the muffins: in a medium bowl, whisk together flour, sugars, baking powder, salt, cinnamon, and nutmeg.

Place butter in a small saucepan, and melt until browned over medium heat. Remove from the heat and cool slightly.

In a medium bowl, whisk together egg, yolk, milk, and vanilla. While whisking, slowly drizzle in the warm butter, making sure to scrape any brown bits into the egg mixture as well. Whisk until well incorporated.

Add the milk mixture to the flour mixture all at once. Fold together with a spatula. Once no flour bits remain, fold in the diced peaches. Divide the batter between the muffin cups.

To make the topping: combine all the ingredients in a small bowl and blend together with your fingers until crumbly. Butter will be the size of oats and small pebbles. Divide the topping among the muffin cups on top of the batter.

Bake muffins for 15 to 18 minutes, or until a skewer inserted into the center of one of the muffins comes out clean. Remove from the oven and cool in the pan 20 minutes before removing. To remove, run a butter knife along the edges of the muffin pan and gently scoop out.