Preheat your oven to 350 degrees F (175 degrees C). Grease a 9×13-inch pan generously with nonstick cooking spray. This step is crucial to ensure your bars come out perfectly.
In a large bowl, whisk together the all-purpose flour, old fashioned oats, baking soda, ground cinnamon, and sea salt. Make sure everything is evenly mixed to avoid clumps of flour.
Using a stand mixer, cream together the unsalted butter, Biscoff cookie butter, light brown sugar, and granulated sugar until smooth, which should take about 2 minutes. Scrape down the sides of the bowl with a spatula to ensure everything is well combined.
Add the eggs and vanilla extract to the mixture and mix until fully incorporated. The batter should be creamy and slightly fluffy.
Gradually add the dry ingredients to the wet mixture. Mix on low speed until just combined. Be careful not to overmix, as this can affect the texture.
Divide the dough in half. Press half of the dough evenly into the bottom of the prepared pan. This will be the base for your bars.
Bake the base for 12 minutes. You want it to be set but not overly browned at this stage.
After 12 minutes, remove the pan from the oven and drizzle the salted caramel sauce evenly over the cookie crust. Make sure to spread it out so every bite gets some caramel goodness.
Top with the remaining dough. You can use a spatula to spread it a little. For easier handling, lightly spray your hands with nonstick cooking spray or dampen them slightly. Gently press the dough down on top of the caramel, ensuring it’s evenly distributed.
Place the pan back in the oven and bake for 24 to 30 minutes, or until the bars are golden brown on top. The edges should look firm and a little crisp.
Once baked, remove from the oven and let the bars cool completely in the pan. This step is important to allow the caramel to set up before cutting into them.