- For the Bars:
- 1/2 cup (1 stick) unsalted butter, melted you could also use browned butter
- 1 large egg
- 1 cup brown sugar, packed (I used light; dark may be used for more robust flavor)
- 2 teaspoons vanilla extract
- 2 teaspoons cinnamon
- heaping 1/3 cup smooth Cookie Butter (or creamy Biscoff; peanut butter will lend a different flavor but may be substituted; or use Homemade Cookie Butter
- 3/4 cup all-purpose flour
- pinch salt, optional and to taste
- For the Streusel Topping:
- 1/4 cup unsalted butter (half of one stick), softened
- 1/2 cup whole rolled old-fashioned oats (not quick cook or instant)
- 1/4 cup brown sugar, packed (I used light; dark may be used for more robust flavor)
- 2 tablespoons granulated sugar
- 2 tablespoons all-purpose flour
- 1+ teaspoon cinnamon, to taste (I used 2 teaspoons, add to taste)
- 2 tablespoons Cookie Butter, melted for final drizzle after baking
For the Bars – Preheat oven to 350F. Line an 8-by-8-inch pan with aluminum foil, spray with cooking spray; set aside. In a large microwave safe bowl, melt 1/2 cup butter, about 1 minute. Allow the butter to cool momentarily so you don’t scramble the egg. Add the egg, 1 cup brown sugar, vanilla, 2 teaspoons cinnamon, and whisk or stir until smooth. Add the heaping 1/3 cup Cookie Butter, and stir to incorporate. Add the flour, optional salt, and stir until just combined; don’t overmix. Pour batter into prepared pan, smoothing the top lightly with a spatula; set pan aside.
For the Streusel Topping – In the same bowl you made the batter in (provided you scraped it fairly clean with a spatula), add all the streusel ingredients except the 2 tablespoons cookie butter for final drizzle post-baking, and work the mixture with a spoon, a pastry cutter, or your hands until small pebbles form. Sprinkle topping evenly over base layer (it looks like there’s quite a bit, but it seeps in during baking).
Bake for about 30 minutes, or until center is set and not jiggly; and the edges are set and have pulled away slightly from sides of pan. It will be hard to determine how set the center is because it’s hard to see what’s going on underneath the streusel topping and edges are the best clues. I baked for 29 minutes, at which point most edges were quite firm, crusted over, and the corners were crisping. Testing for doneness in the center with a toothpick is another clue; toothpick should come out mostly clean.
After bars emerge from the oven, heat 2 tablespoons of Cookie Butter in the microwave to melt, about 15 seconds. Immediately and evenly drizzle Cookie Butter over the top. Allow bars to cool in pan for at least 30 minutes before slicing and serving. Store bars in an airtight container for up to 1 week at room temperature, or up to 3 months in the freezer.