Preheat the oven to 350°F (175°C). Line an 8×8-inch square pan with aluminum foil and spray with nonstick cooking spray (or simply spray the pan directly). Set aside.
In a large microwave-safe bowl, melt the butter in the microwave (about 1 minute).
Whisk the granulated sugar, brown sugar, and vanilla extract into the melted butter.
Add the flour, oats, and a pinch of salt (if using), and stir until well combined. Set aside 1 heaping cup of this oatmeal mixture for the crumble topping.
Press the remaining oatmeal mixture evenly into the bottom of the prepared pan to form the crust.
Spread the raspberry preserves evenly over the crust, making sure to cover the entire surface. The jam layer should be about 1/8-inch to 1/4-inch thick.
Sprinkle the reserved oatmeal mixture evenly over the raspberry layer. For larger crumble chunks, gently squeeze some of the mixture in your hand as you sprinkle it over the top.
Bake for 30 minutes or until the edges are set, and the center is lightly golden and just set.
Remove from the oven and place the pan on a wire rack to cool for at least 2 hours before slicing and serving.