Preheat oven to 350 degrees Fahrenheit.

Cream 1 cup of granulated sugar with the softened butter until light and fluffy.

In a separate bowl, sift together the all-purpose flour, baking powder, and salt.

Add the dry ingredients and milk to the creamed mixture, mixing until well combined.

Pour the batter into a glass baking dish (approximately 1 1/2 quart casserole dish).

Sprinkle the fresh or frozen blackberries evenly over the top of the batter.

In a saucepan, boil 2 cups of water.

Add the remaining 1 cup of granulated sugar to the boiling water and stir until dissolved.

Carefully pour the sugar water over the blackberries.

Bake in the preheated oven for approximately 50 minutes, or until the top is golden brown.

Remove from oven and let cool slightly before serving.

Serve at room temperature or chilled, with ice cream or a dollop of whipped cream.