In a medium bowl, prepare the streusel topping by stirring together brown sugar, flour, and salt. Use a pastry blender (or two knives) to cut the butter into the flour mixture until it resembles small pebbles or sand. Set aside.
Preheat oven to 350 degrees F. Lightly grease a muffin tin, or line with paper cups.
Make your muffin batter in a mixing bowl by beating together butter, brown sugar, sour cream, granulated sugar, vanilla, and eggs using an electric hand mixer until light and creamy. Stir in flour, baking powder, and baking soda until combined.
Spoon batter into prepared muffin tin (makes 24 muffins). Sprinkle a heaping spoonful of streusel on top.
Bake in the preheated oven for about 25 minutes, until a toothpick inserted into the center comes out clean. Remove muffins from tin and transfer to a wire rack to cool completely.