Add the berries and rhubarb to a large mixing bowl.
Combine flour, cornstarch, sugar, and vanilla, with the strawberries and rhubarb. Stir and set aside.
Unroll the crust on a piece of parchment paper and use a round cookie cutter to circles in the pie dough. Gather the dough scraps and re-roll to cut out the remaining dough rounds.
Press the dough into the muffin cups of a well-greased pan.
Spoon the fruit mixture into each of the mini pies. Add a piece of cold butter to the top of each pie filling.
Lay out the other crusts and cut them into 1/4″-3/8 inch strips using a pizza cutter or pastry cutter.
Create a lattice top over each of the pies. Use a pastry brush to brush the crust edges of the circle to seal and crimp edges.
Wrap dough strips around the perimeter of the tops of the pies for the extra crust. Bake for 20-25 minutes or until the filling is bubbly and the tops have a golden color.
Remove from the oven and cool on a wire rack. Serve with vanilla ice cream or a dollop of whipped cream!