Begin by lining your muffin tin with liners, or greasing each cup very well. Preheat the oven to 400°F.

In a mixing bowl, beat together the very soft stick of butter and 1 cup of sugar until well combined. Add the two eggs one at a time, mixing well after each addition. Stir in the 1 cup of buttermilk, the 2 tsp. of vanilla extract, and the 1/4 cup of sourdough starter.

In a separate bowl, stir together the 2 1/4 cups flour, 1/2 tsp. salt, and 2 tsp. baking powder. Add the dry ingredients to the wet and stir until just combined.

Stir in any mixins you want. Divide the batter into the muffin cups. Bake the muffins for 15-18 minutes, or until the edges are golden brown and a toothpick inserted in the center comes out clean or with just a few moist crumbs.

Allow the muffins to cool for a few minutes, then remove them from the muffin pan and let them cool completely.

Store the muffins in an airtight bag or container.