Step 1: Preheat the Oven

1. Preheat your oven to 350°F (175°C). Grease an 8×8-inch baking dish or line it with parchment paper.

Step 2: Prepare the Raspberry Filling

1. In a medium bowl, combine the raspberries, sugar, cornstarch, lemon juice, and vanilla extract. Gently toss to coat the raspberries and set aside.

Step 3: Make the Oatmeal Crumble Topping

1. In a large bowl, mix the rolled oats, flour, brown sugar, baking powder, and salt.
2. Pour the melted butter into the dry ingredients and stir until crumbly and well combined.

Step 4: Assemble the Bars

1. Take about two-thirds of the crumble mixture and press it firmly into the bottom of the prepared baking dish to form the base.
2. Spread the raspberry filling evenly over the crumble base.
3. Sprinkle the remaining crumble mixture over the raspberry filling.

Step 5: Bake

1. Bake in the preheated oven for 25-30 minutes, or until the topping is golden brown and the filling is bubbly.

Step 6: Cool and Serve

1. Allow the bars to cool in the baking dish for at least 15 minutes before slicing.
2. Cut into squares and enjoy!