For Blackberry Sauce:1. Set the blackberries in a strainer over a bowl to defrost. Reserve 1/2 cup of the juice. (Discard the remaining juice or save it for another use).2. Combine the cornstarch and water in a small bowl and stir to dissolve.3. Combine the 1/2-cup reserved juice and the sugar in a medium saucepan, and bring to a boil. Stir in the dissolved cornstarch and continue to cook until the mixture is thickened, 3 to 4 minutes.4. Gently stir in the defrosted blackberries, being careful not to break them up. Taste for sugar and add more if necessary. Scrape the sauce into a bowl or container, and cool to room temperature. Covered and refrigerated, the sauce will keep for 2 or 3 days.For Cobbler Dough:1. Separate the egg yolks from the whites and set the whites aside for another use. Push the yolks through a fine-mesh strainer into a small bowl and set aside.2. Combine the flour, sugar, baking soda, cream of tartar and salt in the bowl of an electric mixer fitted with the paddle attachment. Add the butter and mix on low speed until the mixture resembles coarse meal.3. Add the egg yolks and the heavy cream, and mix until the dough just comes together. Cover the bowl with a kitchen towel until ready to use, up to 1 day.To make Peach-Blackberry Cobbler:1. Preheat oven to 375 degrees.2. Peel, pit and slice the peaches. Place them in a shallow bowl and toss with the lemon juice.3. Sprinkle the cake crumbs over the bottom of an 11 x 7-inch baking dish. Arrange peach slices on top of the crumbs. Cover peaches with Blackberry Sauce.4. Turn the dough out onto a lightly floured work surface and sprinkle it with a little flour. With a lightly floured rolling pin, roll the dough out to form a rectangle a little bigger than the baking dish, rotating the dough as you roll it and occasionally sliding a flat metal spatula underneath to make sure the dough does not stick to the work surface.5. Trim the dough to same size as baking dish. Slide the dough onto a baking sheet. Brush with the melted butter, and sprinkle with the cinnamon sugar. With a sharp paring knife, score the dough with lengthwise and crosswise cuts about 1/16 inches deep to make a pattern of little squares.6. Place the dough in the oven and back until golden brown and firm, about 25 minutes. Remove the crust from the oven, and let it cool on the baking sheet on a wire rack for 10 minutes. Turn the heat down to 250 degrees.7. Place the slightly cooled crust on top of the fruit in the baking dish. Place the dish in the oven and bake until the fruit is warmed through, about 30 minutes. 8. Serve warm, with whipped cream or vanilla ice cream.