Preheat your oven to 180°C (356°F). Line a 9-inch square baking pan with parchment paper, leaving overhang for easy removal.
In a large mixing bowl, whisk together the flour, sugar, salt, and baking powder until combined.
Add the cold, cubed butter to the dry mixture. Use a pastry cutter or your fingertips to quickly rub in the butter until the mixture resembles coarse crumbs with some pea-sized bits.
Press about two-thirds of this crumb mixture evenly into the bottom of the prepared pan, pressing firmly to create a smooth, compact crust.
In a small bowl, combine the raspberries with the sugar and lemon zest, gently stirring to coat the berries evenly. Spread this vibrant mixture over the crust layer.
Sprinkle the remaining crumble mixture evenly over the raspberry layer, pressing lightly into the berries to help everything stick together.
Bake in the preheated oven for 35-40 minutes, until the topping turns a golden brown and the raspberry filling is bubbling at the edges.
Remove the pan from the oven and let the bars cool on a wire rack for at least 30 minutes. This allows the filling to set and the topping to become crisp.
Once cooled, lift the bars out of the pan using the parchment overhang. Slice into squares using a sharp knife.
Serve the bars slightly warm or at room temperature. Enjoy the crisp topping with the jammy, tart raspberry filling for a nostalgic yet sophisticated treat.
