In a small bowl, combine the flour, sugar, salt, and butter.

Stir together until the mixture is well combined and there are small to medium sized chunks. Set aside.

Preheat the oven to 375 F. Line the muffin pan with cupcake wrappers. Set aside.

In a medium bowl, whisk together the flour, granulated sugar, baking powder, and salt.

In a separate bowl, whisk together the egg, sour cream, milk, and vanilla.

Add the egg mixture to the flour mixture and stir until smooth. Add the butter and stir until combined. Fold the blueberries into the batter.

Divide batter among muffin cups, filling them to the top.

Sprinkle a generous amount of streusel topping on each muffin.

Bake the muffins for 15 minutes. Remove from the oven and gently press three blueberries into the top of each muffin to make a Mickey Mouse design. Bake muffins for another 13 minutes until golden brown and a toothpick inserted in the center comes out clean.

Let the muffins cool in the pan for 5 minutes, then transfer to a cooling rack to cool completely.