In a large mixing bowl, combine the flour, baking powder and salt. Add the chunks of cold butter and mix the butter into the flour using a pastry blender. Continue to do this until the mixture looks like bread crumbs or small pieces of butter. Add the cold water and stir it into the mixture so that it all sticks together. Clump the dough all together to form a ball. Wrap in plastic wrap and refrigerate for 1-2 hours. You want the dough chilled and cold, but not hard.
Roll out the dough to a 13 inch circle on a lightly floured surface. Save some dough for your top lattice strips. Lay the circle of crust dough in your greased pie dish. Roll out the remaining dough and cut into 1 inch wide strips for the lattice top and leave aside until fruit in the pie crust.
Whisk Clabber Girl Corn Starch together with 1/4 cup water until well blended. Combine with the rhubarb and sugar in a medium pot and heat until thickened. Then add to strawberries. Add the vanilla and stir all together. Then let sit for about 30 minutes for your filling to thicken. Pour the fruit mixture into the pie crust. Add the butter chunks on top.
Arrange the crust strips in a lattice pattern over the fruit mixture and press the strip ends into the crust. Brush the egg whites onto the top of the crust strips and sprinkle with sugar.
Bake at 400 degrees F for about 35 minutes or golden in color. Let cool for filling to settle and thicken.
Recipe submitted by Stacy Walker - The Backroad Life