Center a rack in the oven and preheat oven to 375 degrees Butter or spray the 12 molds in a regular-size muffin pan or line with paper cups. Place the muffin pan on a baking sheet.
To make the streusel, put the flour, brown sugar, and cinnamon in a small bowl and whisk to combine. Add the bits of cold butter and toss, pinching the mixture between your fingers, until you have pea-sized crumbs. Set aside in the refrigerator.
To make muffins: mix together flour, sugar, brown sugar, baking powder, cinnamon and salt. Whisk well. In a separate bowl combine melted/cooled butter, eggs, milk and vanilla, beat well. Slowly add wet ingredients to dry ingredients. Whisk gently or fold only until combined. Don’t overmix.
Divide the batter evenly among the muffin cups. Sprinkle streusel over each muffin, gently pressing to secure streusel to batter.
Bake for about 18-20 minutes, or until the tops are golden and a toothpick inserted into the center of the muffins comes out clean. Transfer the pan to a wire rack and cool for 5 minutes before carefully removing each muffin. Serve warm or at room temperature. Muffins can be stored, in an airtight container, at room temperature for a couple of days. Serve warm with butter and cinnamon sugar or peach jam.