Prepare Your Tin and Oven: Preheat your oven to 180°C (160°C fan). Grease and line a 20cm x 30cm (8x12 inch) rectangular baking tin with baking parchment, leaving some overhang on the sides to make it easy to lift the bars out later.

Make the Crumble Mixture: In a large bowl, combine the plain flour, caster sugar, rolled oats, baking powder, and salt. Give it a good whisk to mix everything together evenly.

Add the Butter: Add the cold, cubed butter to the dry ingredients. Using your fingertips, rub the butter into the flour mixture until it resembles coarse breadcrumbs with some larger, pea-sized lumps remaining. This texture is what gives the bars their character. What works best for me is to work quickly so the butter doesn't warm up too much. Stir in the vanilla extract.

Form the Base: Take about two-thirds of the crumble mixture and press it firmly and evenly into the bottom of your prepared baking tin. Use the back of a spoon or the bottom of a glass to compact it down into a solid layer.

Prepare the Filling: In a separate bowl, gently mix the raspberry jam, fresh or frozen raspberries, and lemon juice. If you're using frozen raspberries, toss them in the cornflour first before adding them to the jam. This helps to thicken any extra liquid they release.

Assemble the Bars: Spread the raspberry mixture evenly over the pressed crumble base, leaving a small 1cm border around the edges to prevent the jam from catching and burning.

Add the Topping: Sprinkle the remaining one-third of the crumble mixture evenly over the raspberry layer. Don't press it down; let it remain loose and crumbly.

Bake to Perfection: Place the tin in the preheated oven and bake for 40-45 minutes. You'll know it's ready when the topping is a lovely golden brown and the raspberry filling is bubbling gently at the edges.

Cool Completely: This is a crucial step! Allow the bars to cool completely in the tin before even thinking about slicing them. I usually let them cool on a wire rack for at least 2 hours. This allows the filling to set, ensuring you get clean, neat slices.