Blackberry-Peach Cobbler with Sour Cream Biscuits

For the biscuits:

1. 1
   Combine flour, brown sugar, baking powder, baking soda, and salt in a mixing bowl and whisk to aerate and break up any lumps.
2. 2
   Using the large holes of a box grater, grate the frozen butter into the flour mixture and toss to coat. Smear in the sour cream and heavy cream and, using your hands, knead until the dough just comes together. Divide into six pieces and flatten into disks; cover and reserve in the refrigerator.

For the filling:

1. 1
   In a large bowl, mix together flour, brown sugar, granulated sugar, salt, and vanilla seeds. Add peaches and blackberries and mix until the fruit is evenly coated.
2. 2
   Turn the fruit mixture into a 2-quart baking dish and stagger the reserved biscuit dough evenly across the top.

To assemble:

1. 1
   Brush the tops of the biscuits with sour cream and sprinkle with sugar.
2. 2
   Bake until the filling is bubbling, the peaches are tender when pierced with a knife, and the biscuits are golden brown and cooked through, about 30 to 35 minutes. Let cool at least 5 minutes before serving.