Preheat the oven to 350°F (175°C). Grease and flour the cake pan.

In a mixing bowl, whisk together the dry ingredients.

In another bowl, cream the butter and sugar until light and fluffy.

Add the eggs and vanilla extract, mixing until combined.

Stir in the sourdough starter and mix well.

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

Gently fold in the boysenberries.

Pour the batter into the prepared cake pan and smooth the top.

Bake for 40-45 minutes or until a toothpick inserted into the center comes out clean.

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