Preheat the oven to 375°F and line a 12-cup muffin pan with paper liners. Prepare the crumb topping by mixing flour, brown sugar, cinnamon, salt, and cold butter until crumbly, then chill it while making the batter.
Whisk together the flour, baking powder, baking soda, and salt in a large bowl so the leavening ingredients distribute evenly for consistent muffin texture.
In another bowl, whisk melted butter and sugar until smooth, then mix in eggs, sour cream, and vanilla extract until creamy and fully combined.
Fold the wet ingredients into the dry ingredients gently until just combined. Avoid overmixing so the muffins stay soft and fluffy after baking.
Fill muffin liners halfway with batter, sprinkle cinnamon sugar filling, then top with remaining batter and a generous amount of crumb topping.
Bake for 20–22 minutes until golden brown and a toothpick inserted into the center comes out clean. Cool slightly before serving or glazing.