Preheat oven to 350 degrees. Place butter in a 12-inch cast iron skillet and melt butter in preheating oven, takes about 5 minutes. Remove pan from oven when butter is melted and set aside.
In a large bowl, mix sugar, flour, baking powder, and salt. Whisk together.
Add milk, sourdough discard, and vanilla and mix by hand until just incorporated. Do not over mix.
Pour cobbler batter into the pan over the melted butter. Do not mix. Top with the blueberries and sprinkle the brown sugar on top.
Bake for 40 minutes at 350 degrees. The top should be golden brown. Allow to cool for 20 minutes then serve with ice cream or store at room temperature.
Notes
You can use a different size cast iron skillet if you want but the fruit may end up very close together which will increase baking time a bit.