Preheat the oven to 300°F (149°C). Line the bottom and sides of an 8-inch square baking pan with parchment paper, leaving enough overhang on all sides. Set aside.

Stir the melted butter, granulated sugar, vanilla, and salt together in a medium bowl. Add the flour and stir until everything is combined. Press the mixture evenly into the prepared baking pan. Bake for 15 minutes.

Remove the crust from the oven, and turn the oven up to 350°F (177°C).

Spread preserves over warm crust. Dot the preserves with raspberries on top.

Whisk the oats, brown sugar, cinnamon, and flour together in a medium bowl. Cut in the chilled butter with a pastry blender or two forks (or even with your hands) until the mixture resembles coarse crumbs. Sprinkle the filling with streusel and bake for 30–35 minutes or until the streusel is golden brown. The raspberry filling should be bubbling on the edges.

Remove from the oven and allow to cool for at least 20 minutes at room temperature, then chill in the refrigerator for at least 2 hours (or overnight). Lift the foil or parchment out of the pan using the overhang on the sides and cut into bars. I usually cut them into 16 smaller bars, but you can cut them into 12 larger bars. Drizzle the bars with vanilla icing, if using. These raspberry bars can be enjoyed at room temperature or cold. I personally like them cold.