Cobbler is all about the fruit-to-dough ratio. You need a light, doughy top but it should never overshadow the star of the dish: fresh, delicious, baked, slightly sugared fruit. If you're worried about an expensive item like blueberries, err on the side of a smaller cobbler like this one.
Note: Both the filling and the dough can be made up to a day in advance.
Preheat the oven to 400 degrees.
Combine the first 3 ingredients plus ¼ cup of the sugar in a bowl together with the mint (if using), and toss lightly until all ingredients are mixed. Grease an 8-by-12-inch pan with butter and pour in the mixture.
In another bowl, combine the flour, baking powder, cinnamon, salt, and remaining sugar together. Add the butter in small chunks, without melting, and mix together with a fork or a handheld mixer on low speed. Mix in the egg, taking care not to overmix.
Add the dough to the top of the cobbler filling in spoonfuls. The dough does not need to be spread evenly since it will flatten as it bakes.
Place in the oven for 30-40 minutes until the top is browned. Remove from the oven, and serve.