1. Preheat the Oven: Start by preheating your oven to 375°F (190°C). Line a muffin tin with paper liners or grease it lightly to ensure the muffins release easily.

2. Mix the Dry Ingredients: In a large bowl, whisk together the all-purpose flour, baking powder, salt, and granulated sugar.

3. Mix the Wet Ingredients: In another bowl, beat the egg, then add the milk, vegetable oil, and vanilla extract. Stir until well combined.

4. Combine the Batter: Pour the wet ingredients into the dry ingredients and stir until just combined. Gently fold in the diced peaches.

5. Prepare the Crumb Topping: In a small bowl, mix the brown sugar, all-purpose flour, and ground cinnamon. Add the melted butter and create a crumbly texture.

6. Fill the Muffin Cups: Divide the muffin batter evenly among the muffin cups, filling them about two-thirds full. Sprinkle crumb topping over each muffin.

7. Bake to Perfection: Bake for 18-22 minutes, or until a toothpick comes out clean.

8. Cool and Serve: Allow the muffins to cool for 5 minutes, then transfer to a wire rack to cool completely.