Heat the oven to 400 degrees F, and combine the flour, sugar, baking powder, and salt in a large bowl and whisk or stir with a fork to mix well..

Chop the butter into small bits, toss with flour mixture, and then use a pastry blender, two knives, or your hands to work most but not all of the butter into the flour, pinching and squeezing to made a raggedy, lumpy mixture with pea-sized bits of butter still showing.

Add most of the milk and use your hand or a big spoon to transform the dry mixture into a raggedy dough.

Turn it out onto a flour counter or cutting board and knead gently about 5 turns, until you have a nice soft dough, dipping hands in flour to help sticky goo join the dough.

Set the dough on a large, floured sheet of baking parchment, big enough to line an 11 x 17 sheet pan.

Roll it out into a rectangle about 10 by 12 inches and about ¼ inch thick.

Pour half the melted butter over the center portion of the dough, leaving a 1 inch border.

Scatter the berries over the buttered dough, and arrange them somewhat evenly, leaving a 1-inch border all around the edges.

Sprinkle both sugars evenly over the buttered dough and berries. Pour the remaining half of the melted butter over the sugared berries.

Reaching up and across the berry covered dough to the far edge, gently roll the dough toward you, enclosing the sugared berry filling and making a big fat roll. Roll it into a cylinder as tightly and evenly as you can, using the parchment paper to help you manage the unwieldy shape.

Stop rolling when the open edge is on the bottom, and let this be the center seam underneath the roll.

Carefully transfer the roll on its parchment paper bed to the baking sheet, and bake at 400 degrees until the roly poly is nicely browned and firm, and the doughy center of the roll is cooked through, about 30 minutes.

OR slice the roly ply into 1 inch thick rounds and transfer to a well-buttered 9-inch round cake pan; or a spring form pan; wrapping the base of spring form pan well with aluminum foil to catch any syrup.

Meanwhile, make the blackberry sauce, combining the 2 cups blackberries, water, and sugar in a medium saucepan and bring to a boil over medium-high heat.

Stir well and boil gently for 5 to 7 minutes, using a potato masher or a large metal slotted spoon to mash berries well..

When they have released their juices and formed a lightly thickened sauce, set aside to cool.

When the roly poly is ready, remove it from the oven and cool for at least 10 minutes; then let cool to room temperature.

Slice or scoop out portions, hot, warm, or at room temperature, and serve in bowls, anointed with the blackberry sauce, and if possible, topped off with a flourish of ice cream or whipped cream.

Note: To remove the seeds, press the berry mixture through a fine-mesh strainer over a bowl and scrape the rounded outside of the strainer to capture every drop of the rich, sweet berry sauce.