Beat butter until soft and creamy. Add brown sugar and granulated sugar, and cream together with a mixer.
½ cup butter, ⅓ cup brown sugar, ¼ cup granulated sugar
Beat in yogurt, eggs, and vanilla.
2 eggs, ½ cup Greek yogurt, 2 teaspoon vanilla
Add flour, baking powder, baking soda, salt, cinnamon, and nutmeg to the top of the batter. Begin mixing the dry ingredients together first, then mix into the wet ingredients. Stir just until most of the flour is incorporated.
Add blueberries and chopped peaches. Stir just until the fruit is well distributed, and any remaining flour is mixed into the batter.
1 cup blueberries, 1 cup peaches
Scoop into muffin tins that have been lightly greased or lined with paper liners.
Sprinkle with cinnamon sugar.
1 tablespoon cinnamon sugar
Bake in a preheated 375°F oven for 18-22 minutes until the muffins are fully cooked and golden brown. Use a toothpick inserted in the center of a muffin to determine if they are fully cooked. If the toothpick comes out with just a few crumbs and no wet batter, the muffins are done.
Allow muffins to cool for a few minutes. Then remove from muffin tin and continue cooling on a baking rack.
Notes
Measure the flour carefully. Use the fluff and scoop method so the muffins stay light and tender.
Do not overmix the batter. Stir just until the dry ingredients are incorporated. Overmixing can make muffins dense.
Cut peaches into small, even pieces. Smaller pieces distribute better and help the muffins bake evenly.
Gently fold in the fruit. This helps keep the blueberries from breaking and turning the batter purple.
Fill the muffin cups evenly. A cookie scoop or measuring cup makes it easy to portion the batter so all muffins bake at the same rate.