Preheat and Prepare: First things first, preheat your oven to 180°C (160°C fan). Grease and line a 20cm (8-inch) square baking tin with baking parchment, leaving some overhang on two sides to act as handles. This will make lifting the bars out later much easier.

Mix the Dry Ingredients: In a large mixing bowl, whisk together the plain flour, caster sugar, baking powder, and salt. This ensures everything is evenly distributed before we add the butter.

Cut in the Butter: Add the cold, cubed butter to the flour mixture. Using your fingertips, a pastry blender, or a food processor on a pulse setting, cut the butter into the flour until the mixture resembles coarse breadcrumbs with some pea-sized pieces of butter still visible. This process is key to a tender crumb, as explained in this helpful guide to making shortbread.

Form the Dough: In a small bowl, lightly beat the egg with the vanilla extract. Pour this into the flour and butter mixture. Use a fork or your hands to bring it together until it starts to form a clumpy, shaggy dough. Don't overmix it; we want to keep it crumbly.

Create the Base: Take about two-thirds of the dough mixture and press it firmly and evenly into the bottom of your prepared baking tin. Use the back of a spoon or your fingers to create a flat, compact layer.

Add the Topping Ingredients: Add the rolled oats to the remaining one-third of the dough in the bowl. Gently toss with your fingers to combine. This will be your crumble topping. Adding oats gives the top a lovely chewy texture and helps it brown nicely.

Layer the Filling: If you're using it, stir the lemon juice into your raspberry jam. Spoon the jam over the base and spread it out into an even layer, leaving a small 1cm border around the edges to prevent the jam from catching and burning on the sides of the tin.

Add the Crumble and Bake: Sprinkle the remaining oat and dough mixture evenly over the jam layer. It doesn't need to be perfect; a rustic, uneven crumble looks best. Place the tin in the preheated oven and bake for 35-40 minutes, or until the top is a lovely golden brown and the jam is bubbling at the edges.

Cool Completely: This is a crucial step! Remove the tin from the oven and place it on a wire rack to cool completely. I find that leaving it for at least 2 hours is best. If you try to cut the bars while they're warm, they will fall apart. Patience is key here. Once fully cooled, use the parchment handles to lift the bake from the tin, place it on a cutting board, and slice it into 16 squares.