Pre-heat oven to 350 degrees.

Melt the butter and brown sugar over medium heat in a large saucepan. Add blackberries and toss to coat. Let cook 1 minute.

Remove from heat and pour into a 9-inch cake pan sprayed with cooking spray.

Beat the sugar and butter together in a bowl with an electric mixer until light and fluffy. Beat in the eggs.

Place the flour, baking powder, and salt in a medium-sized bowl and stir to combine.

Add the vanilla extract to the milk and alternately stir the flour and milk into the butter mixture, beginning and ending with flour.

Spoon the batter over top of the blackberry mixture in the cake pan.

Bake the cake in a preheated oven for 35 minutes, or until a toothpick comes out clean when inserted into the center of the cake.

Let the cake cool on in pan on a rack for 30 minutes.

Loosen the cake from pan by sliding a knife along the inside edge of the pan. Place the cake plate over top of the pan and flip. Lift the pan slowly to release the cake.