Place 2 tablespoons of melted butter, 1/4 cup of sugar, and 1/4 cup of flour in a small bowl. Use a fork to mix it until it becomes a crumble. Then set it aside.
Preheat the oven to 350°F and prepare a 12-count muffin pan with muffin liners or by greasing it with softened butter or cooking spray.
In a large bowl combine 2 cups of flour, 1 cup of sugar, 2 teaspoons of baking powder, 1/2 teaspoon baking soda, and 1/2 teaspoon of salt.
Add flavor variations to the flour mixture. See note for ideas.
In a medium bowl, whisk together 1 stick of melted butter, 2 eggs, 1/2 cup of sourdough discard, 1/4 cup of milk, 2 heaping tablespoons of sour cream, and 1 teaspoon of vanilla until smooth.
Pour the wet ingredients into the dry and stir until combined.
Divide the batter evenly between 12 muffin cups. Then sprinkle the crumb mixture on top. Gently press the crumble on top of the batter.
Bake muffins in a preheated oven for 30-35 minutes until the top of the muffin springs back when you press it or a toothpick comes out clean.
Let the muffins cool on a wire rack.