Strawberry-Rhubard Cobbler

This recipe is from the America's Test Kitchen Family Cookbook
For the Filling:
1 qt strawberries, rinsed, hulled and halved (if berris are large)
10 ounces rhubard, cut into 1/2 inch chunks
1 tablespoon cornstarch
1/2 sugar
1 teaspoon vanilla extract
For the Topping:
1 cup all-pourpose flour
1/4 cup, plus 2 teaspoons sugar
2 teaspoons baking powder
1/4 teaspoon baking soda
1/4 teaspoon salt
1/3 cup buttermilk
4 tablespoons unsalted butter, melted and cooled
1/2 teaspoon vanilla extract
1/8 teaspoon cinnamon

1. Mix together the fruit, cornstarch, sugar and vanilla. Set aside.
2. Adjust the oven rack to the middle position and heat the oven to 400 degrees. Place the fruit filling in a 9-inch deep-dish pie plate. Place the pie plate on a foil-covered, rimmed baking sheet (this will keep the drippings off the bottom of your oven) and bake until the fruit begins to release liquid, about 20 to 30 minutes.
3. While the friut filling is baking, whisk the flour, 1/4 cup sugar, the baking powder, baking soda, and salt together in a large bowl. In a seperate bowl, whisk the buttermilk, melted butter, and vanilla together. In a third bowl, toss the cinnamon together with the remaining 2 teaspoons of sugar.
4. Once the fruit filling has begun to reliase liquid, gently stir the buttermilk mixture into flour mixture with a rubber spatula until the dough is just combined and no dry pockets remain.
5. Remove the cobbler filling from the oven and stir. Pinch the biscuit dough into 8 equal pieces and place them on top of the hot filling, spaced 1/2 inch apart. Sprinkle the dough with the cinnamon sugar.
6. Return the cobbler to the oven and bake until the filling is bubbling and the biscuits are golden brown on top and cooked through, 15 to 20 minutes. Let the cobbler cool on a wire rack for 15 minutes before serving.