Step 1: Make the Shortbread Crust

1. Preheat oven to 350°F (175°C). Grease and line an 8×8-inch baking pan with parchment paper.
2. In a mixing bowl, beat butter and sugar until light and fluffy.
3. Add flour, salt, and vanilla extract, mixing until the dough is crumbly.
4. Reserve ¾ cup of the dough for the crumble topping.
5. Press the remaining dough firmly into the bottom of the pan.
6. Bake for 12-15 minutes, or until lightly golden.

Step 2: Make the Strawberry Rhubarb Filling

1. In a saucepan over medium heat, combine rhubarb, strawberries, sugar, and lemon juice.
2. Cook for 5-7 minutes, stirring occasionally, until the fruit softens.
3. In a small bowl, mix cornstarch with 1 tablespoon water, then stir into the fruit mixture.
4. Cook for 1-2 more minutes until thickened. Remove from heat and stir in vanilla extract.
5. Let cool slightly before spreading over the baked shortbread crust.

Step 3: Assemble & Bake

1. Crumble the reserved shortbread dough evenly over the fruit filling.
2. Bake for 30-35 minutes, or until the topping is golden brown.
3. Let cool completely before slicing into bars.

Step 4: Serve & Enjoy

1. Optionally, dust with powdered sugar or serve with whipped cream or ice cream.
2. Store leftovers in an airtight container at room temperature for 2 days or in the fridge for up to 5 days.