Preheat the oven to 350°F (175°C) and grease the muffin tin.

In a large mixing bowl, whisk together the sourdough starter, melted butter, eggs, milk, and vanilla extract until smooth.

In another bowl, combine the flour, sugar, baking powder, baking soda, and salt.

Gradually add the dry ingredients to the wet ingredients, mixing until just combined.

In a separate bowl, mix together the flour, brown sugar, and cinnamon.

Add the cold butter and use a fork or pastry cutter to mix until crumbly.

Scoop the muffin batter into the prepared muffin tin, filling each cup about 2/3 full.

Sprinkle the streusel topping evenly over each muffin.

Bake in the preheated oven for 20-25 minutes, or until a toothpick inserted into the center comes out clean.

Allow to cool in the pan for 5 minutes before transferring to a wire rack to cool completely.