In a medium mixing bowl, combine the melted butter, flour, sugar, cinnamon, and salt. Use a fork to mix everything together until large, crumbly clumps form. Set aside while you prep the muffins.

Preheat your oven to 425°F and line a standard muffin tin with paper liners.

In a large bowl, whisk together both flours, sugar, baking powder, and salt until evenly combined.

In a separate smaller bowl, stir together the Greek yogurt, melted butter, milk, eggs, and vanilla extract until smooth.

Dice the peach into small ¼-inch cubes. Grate the zucchini, then place it in a fine-mesh strainer set over a bowl to drain any excess moisture.

Pour the wet ingredients into the dry and gently fold everything together with a spatula. The batter will be thick and a bit lumpy.

Add the peaches and drained zucchini to the bowl, and fold just until everything is incorporated. Try not to overmix—this helps keep the muffins tender.

Scoop the batter into the prepared muffin pan, filling each cup nearly to the top. Generously sprinkle the crumb topping over each muffin.

Bake for 5 minutes at 425°F, then (without opening the oven) reduce the temperature to 350°F and continue baking for another 12 to 16 minutes. The muffins are done when a toothpick inserted in the center comes out clean or with a few moist crumbs.