Preheat your oven to 350°F (177°C) and line an 8x8-inch baking pan with parchment paper.

In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, brown sugar, baking powder, and salt. Cut in the cold butter until the mixture resembles coarse crumbs, then stir in the vanilla extract.

Press about 2/3 of the crumb mixture firmly into the bottom of the prepared baking pan.

In a separate bowl, combine the diced strawberries, chopped rhubarb, granulated sugar, cornstarch, and lemon juice. Fold the ingredients until well-coated.

Spread the fruit filling over the pressed crust in the pan. Sprinkle the reserved crumb mixture over the top of the filling.

Bake for 40-45 minutes, until golden brown and filling is bubbly. Allow to cool completely before lifting out of the pan.

Once cooled, slice into squares or bars and serve chilled or at room temperature.