Preheat the oven to 350°F (175°C) and grease a 9-inch round cake pan.

In a large mixing bowl, cream together the softened butter and granulated sugar until light and fluffy.

Add the eggs one at a time, mixing well after each addition.

Stir in the sourdough starter, lemon juice, lemon zest, and vanilla extract until combined.

In another bowl, whisk together the flour, baking powder, baking soda, and salt.

Gradually add the dry ingredients to the wet mixture, alternating with the buttermilk, and mix until just combined.

Pour the batter into the prepared cake pan and smooth the top with a spatula.

Bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean.

Allow the cake to cool in the pan for 10 minutes before transferring it to a wire rack to cool completely.