First, preheat your oven to 375 degrees Fahrenheit (190 degrees Celsius) and line a muffin tin with paper liners.
In a small bowl, prepare the streusel topping. Combine the flour, granulated sugar, brown sugar, and cinnamon. Add the cold cubed butter and use a fork or your fingers to cut it into the dry ingredients until crumbly. Set aside.
In a large mixing bowl, whisk together the flour, granulated sugar, baking powder, baking soda, and salt.
In a separate bowl, whisk together the melted butter, buttermilk, vanilla extract, and egg until well combined.
Pour the wet ingredients into the dry ingredients. Gently stir until just combined. Do not overmix. A few lumps are fine.
Spoon the muffin batter evenly into the prepared muffin tin, filling each cup about two-thirds full.
Sprinkle the streusel topping generously over each muffin, pressing it lightly into the batter.
Bake in the preheated oven for 18 to 20 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Remove the muffins from the oven and allow them to cool in the muffin tin for a few minutes. Then, transfer them to a wire rack to cool completely.
Once cooled, serve coffee cake muffins. If desired, frost with some vanilla glaze!