Preheat the oven to 350 degrees F and line a 9x13 inch baking pan with parchment paper.
In the bowl of a stand mixer, combine the flour , powdered sugar and softened butter.
Mix on low speed until a dough forms.
Transfer the dough to the prepared pan and press it down towards the edges of the pan until all of the pan has been covered evenly with dough.
Place the base into the oven and bake for 20 minutes.
While the base is baking, prepare the filling.
Add the sliced strawberries and rhubarb to a large bowl.
Add the sugar, vanilla and cornstarch and gently mix through the fruit.
When the base is ready take it out of the oven and using a slotted spoon, transfer the fruit to the base.
Spread the fruit out evenly.
In another bowl, combine the ingredients for the topping except the butter and mix well.
Grate the butter into the mixture then rub the butter with your fingertips into evenly sized pebbles of butter.
Sprinkle the topping over the fruit and place back into the oven.
Bake for 45-50 minutes and the crumb topping is golden.
Let the bars cool before slicing into squares.