Start by melting 1 1/4 cups of butter in a microwave or on the stovetop until it’s completely melted. In a large mixing bowl, combine the melted butter with quick oats, 2 1/4 cups of flour, and 1 1/2 cups of brown sugar. This mixture will form the deliciously crumbly crust for your bars. Once combined, press approximately two-thirds of this mixture firmly and evenly into the bottom of a greased 9×13 inch baking pan. This sturdy base will hold everything together once baked, so ensure it’s compact.
Preheat your oven to 325ºF and pop the pan with your crust mixture into the oven. Bake for 10 minutes; this brief baking time will set the crust nicely without overcooking it. Once done, allow the crust to cool sufficiently while you prepare the caramel filling.
In a saucepan over medium heat, melt 1/4 cup of butter. Stir in 1 cup of brown sugar, mixing until it’s completely smooth, which should take about a minute. Gradually add in the 1/4 cup of milk, 2 tsp of vanilla extract, and 2 tbsp of flour, stirring constantly. Be patient as you heat this mixture — it should come to a slow boil and thicken after about 3 minutes. Continuing to stir ensures a smooth caramel texture without any lumps.
Once the crust is cool to the touch, pour the luscious caramel filling evenly over the baked crust, spreading it out flat with a spatula. Now take the remaining one-third of the reserved crust mixture and sprinkle it generously over the caramel layer. This creates a wonderful contrast in texture. Next, lower the oven temperature to 300ºF and bake the assembled bars for an additional 20 minutes. Although the mixture may seem runny when you take it out, rest assured that it will solidify as it cools completely.
Notes
Let the bars cool completely before cutting; this will make slicing much easier.