• Preheat the oven to 300°F. Line the bottom and sides of an 8"x8" square baking pan, leaving a bit of overhang on each side.

Crust:

• In mixing bowl, beat the butter and sugar until combined.
• Wtih mixer on low, beat in the flour, cornstarch and salt until crumbly.
• Press the mixture firmly into the bottom of the prepared pan, and bake for 20 minutes.
• Remove crust from oven, and turn oven up to 350°F.

Strawberry Filling:

• In a medium-sized bowl, toss together the sliced strawberries, sugar, cornstarch, and lemon zest (if using) until the straberries are coated.

Crumble Topping:

• In another medium-sized bowl, mix together the flour, brown sugar, granulated sugar, and salt until evenly combined.
• Stir in the melted butter. Mix until combined.

Assembly:

• Spoon the strawberry mixture over the crust until it's evenly covered, discarding any juices left in the bowl.
• Cover the berries with the crumble mixture.

Bake & Serve:

• Bake for 30-35 minutes until the crumble topping turns a slight golden brown the strawberry filling is bubbling through.
• Remove from the oven and allow to cool completely before slicing into bars.