Preheat your oven to 350°F. Line a 9x13 baking pan with parchment paper or grease it well.

In a medium mixing bowl, blend together butter, sugar, and flour using a pastry cutter or a food processor until the mixture is crumbly. Press this dough into the bottom of the prepared pan and bake for 15 minutes.

As the crust bakes, combine raspberries, sugar, water, and cornstarch in a saucepan. Stir and cook over medium heat for about 5 minutes until the sugar dissolves and the mixture thickens.

Spread the raspberry mixture evenly over the baked crust.

To prepare the streusel topping, mix the flour, brown sugar, oats, sugar, and cinnamon together. Cut in the butter until the mixture is crumbly.

Sprinkle the streusel evenly over the raspberry layer and bake for 30-35 minutes until the top is golden and the raspberry filling is set.

Allow the bars to cool for at least 2 hours. Drizzle with the glaze if desired, made by mixing powdered sugar, milk, and vanilla extract.