Preheat your oven to 350 degrees. Place a stick of butter in a 9x13 baking dish and place the dish in the preheated oven to melt.
In a small bowl, combine the blackberries, 1 1/4 cup sugar, lemon juice and lemon zest. If using frozen blackberries, allow them to come to room temperature before mixing with the other ingredients. Set this mixture aside.
In a large bowl, add the sourdough starter, 1 cup sugar, baking powder, and salt. Whisk until well combined.
When the oven is preheated and the butter is melted, carefully use oven mitts to remove the baking dish from the oven. First pour the sourdough mixture right on top of the melted butter, allowing it to spread out in the pan. Next, pour the blackberries on top of the sourdough mixture.
Place the baking dish in the oven and bake for 45 minutes until the crust is a deep golden brown.
Serve warm with a scoop of vanilla ice cream :)