Melt 1 ¼ cups of unsalted butter in a microwave-safe bowl for 1 to 2 minutes.

In a large mixing bowl, combine the melted butter, 1 ½ cups of quick oats, 2 ¼ cups of all-purpose flour, and 1 ½ cups of brown sugar.

Press two-thirds of the mixture into a greased 9x13-inch baking pan to form the crust.

Preheat oven to 325°F (163°C) and bake the crust for 10 minutes until set and lightly golden.

In a medium saucepan, melt ¼ cup of unsalted butter over low heat and add 1 cup of brown sugar.

Stir in ¼ cup of milk, 2 teaspoons of vanilla extract, and 2 tablespoons of flour until smooth.

Bring the caramel mixture to a gentle boil and simmer for 3 minutes while stirring constantly.

Pour the caramel filling over the cooled crust and spread evenly.

Sprinkle the reserved crust mixture over the caramel layer.

Reduce the oven temperature to 300°F (149°C) and bake for an additional 20 minutes.