Preheat and Prepare: Preheat your oven to 400 degrees. Place the stick of butter in the bottom of a 9x13 baking dish. Pop the baking dish in the oven as it preheats, just long enough for the butter to completely melt then pull the dish back out of the oven.
Mix the Batter: In a medium bowl, combine the sourdough starter, sugar, baking powder and salt. Stir just until combined. The mixture should resemble a pancake batter.
Prep the Rhubarb: Wash and dry your rhubarb stalks and chop them into 1/2 inch pieces and place them in a large mixing bowl. Add in your sugar, lemon juice, and lemon zest. Give them a gentle mix and let them set for a few minutes until the rhubarb starts to release its juices and the sugar is mostly dissolved.
Assemble the Cobbler: Pour the batter over the melted butter in the bottom of the baking dish. Do not stir. Pour the rhubarb filling evenly over the top of the batter. Again, don’t stir! This layering is what helps form that classic cobblestone street appearance of cobblers.
Bake!: Pop the baking dish in the oven and bake for 45-50 minutes or until the cobbler crust is puffed and golden brown and the rhubarb filling is bubbling around the edges.
Cool and Serve: Let the homemade rhubarb cobbler cool for about 10-15 minutes. This helps the cobbler filling set. Serve warm with a scoop of vanilla ice cream or homemade whipped cream!