Preheat the oven to 350°F (175°C). Line an 8×8-inch baking pan with parchment paper and lightly grease it.
Make the crust and crumble: In a large bowl, whisk together flour, sugar, baking powder, and salt. Cut in the cold butter using a pastry cutter or your hands until the mixture resembles coarse crumbs. Add egg yolk and vanilla, mixing until just combined. The dough will be crumbly.
Form the base: Press about 2/3 of the mixture evenly into the bottom of the prepared pan. Use the back of a spoon or your fingers to flatten it well.
Make the filling: In a medium bowl, gently toss raspberries with lemon juice, cornstarch, and sugar until coated.
Assemble: Spoon the raspberry mixture evenly over the crust layer. Sprinkle the remaining crumble mixture on top.
Bake: Bake for 35–38 minutes or until the top is golden brown and the filling is bubbly.
Cool: Let the bars cool completely in the pan before lifting them out and slicing into squares.
Serve & Enjoy! Best served slightly chilled or at room temperature.