Heat a cast iron skillet over medium high heat.

Add the 2 tablespoons of butter and the brown sugar and stir to make a caramel.

Add the blackberries and 3/4 cup sugar, and stir. Press the blackberries with a spatula to break them up and release their juices (you don't want them to cook into a puree, just broken down some.) Depending on how juicy your blackberries are, cook for 3-4 minutes until it reduces slightly and forms a syrup.

In a bowl, whisk the flour, baking soda and salt. In a mixer, add 1/2 cup softened butter and and 1 cup sugar. Mix until the butter is incorporated, then add the eggs. Continue beating and add flour mixture, vanilla and milk a little at a time and beat until all is incorporated.

Pour the batter into the blackberry syrup. Using a spatula, "tease" the batter to the edges of the pan.

Place on a grill running at 350 degrees. Cook for 25-30 minutes or until a toothpick inserted in the middle comes out clean.

Remove from grill. Using a knife, lightly go around the edges to release from the pan. Allow the cake to sit for 10 minutes to allow the cake to tighten up. Position a plate on top of the skillet, and then flip over. Slowly raise pan. (If the cake doesn't release, put the skillet on a stovetop for 3-4 minutes to heat up the syrup again, then try again.)