Heat oven to 325 F.
Line a 9x13 baking pan with aluminum foil, leaving an overhang on each side. Grease with butter or non-stick cooking spray and set aside.
In a large bowl using an electric or hand-held mixer on medium speed, beat together the butter and sugar until well combined.
Add vanilla and beat until combined.
Turn the mixer down to low and beat in the flour until combined.
Press the mixture firmly into the bottom of the prepared pan. Bake for 15 minutes. While it is baking, make the strawberry layer and topping.
To make the strawberry layer toss together the strawberries, cornstarch, sugar and lemon zest in a medium sized bowl. Set aside.
To make the streusel topping, in a medium bowl stir together all the streusel ingredients using a large wooden spoon or rubber spatula. It will be crumbly and that’s okay.
After the bottom layer is done baking, remove it from the oven.
Carefully spoon the strawberry mixture over the shortbread base until it’s evenly covered. Then crumble the streusel layer over the strawberry layer.
Bake for 50-55 minutes until the streusel topping begins to turn golden brown and you can see the strawberry layer bubbling underneath.
Cool completely before cutting into bars. Tip: these will hold up best if you store them in the refrigerator. Enjoy!