Pour in the melted butter and vanilla extract, stirring with a spoon until a uniform dough forms.
Press the dough evenly into the bottom of your prepared pan using clean hands or the back of a spoon. Bake for 15 to 20 minutes until lightly golden brown. Set aside.
Cook the Filling and Make Crumble
In a medium saucepan, combine the cherries, granulated sugar, lemon juice, and cornstarch. Cook over medium heat, stirring frequently, until the cherries release their juices and the sauce thickens into a glossy pie filling (about 5 to 7 minutes). Remove from heat.
In a separate bowl, whisk together the rolled oats, flour, and brown sugar. Pour in the melted butter and stir with a spoon until thick crumble clusters form.
Assemble, Bake, Cool and Slice
Pour the cooked cherry filling evenly over the pre-baked shortbread crust. Sprinkle the oat crumble topping evenly over the cherries.
Bake at 180°C (350°F) for 30 to 35 minutes until the oat topping is golden brown and crisp.
Leave the bars in the pan to cool completely at room temperature for at least 1 to 2 hours. Transfer to the fridge for 30 minutes to firm up the base.
Lift the cooled block out of the pan using the parchment overhang. Use a large, sharp knife to slice into 16 even squares, wiping the blade clean between cuts, and serve!
Notes
Measuring Ingredients: For the best and most accurate results, use a digital kitchen scale to weigh dry ingredients (especially flour) or use the spoon-and-level method with measuring cups.
Pan Material: Use a light-colored metal baking pan for the crispest shortbread base and even baking. Glass or ceramic pans take longer to heat up and may require 3 to 5 extra minutes of bake time.
Lining the Pan: Leave an overhang of parchment paper on two opposite sides of the pan. This creates a sling so you can easily lift the cooled bar slab out of the pan for clean cuts.
Cherries: Fresh sweet cherries yield the best texture. If using frozen cherries, add them straight to the saucepan frozen without thawing first.
Cooling & Chilling: Do not slice the bars while warm, or the fruit layer will be runny. Let them cool completely at room temperature, then chill in the fridge for 30 minutes to firm up the base for clean cuts.
Serving Warm: If you prefer warm bars, slice them while chilled, then reheat individual squares in the microwave for 10 to 15 seconds before serving.