Apple cider blondies are dense and chewy in all of the best ways. Richly flavored with apple cider, brown butter, and apple pie spice, these blondie bars are bound to be your new favorite fall dessert.
Brown the Butter: In a medium, light-colored, heavy-bottomed saucepan, melt the butter over medium heat, stirring frequently.
Once melted, allow the butter to continue cooking over medium heat, stirring constantly. As it cooks, the butter will begin to foam and then subside. Brown specks will begin to form at the bottom of the pan and the butter will take on a nutty aroma.
Once the butter starts to brown, watch it very carefully – it will go from brown to burnt very quickly. As soon as the butter reaches your desired color, immediately remove it from the heat and pour it into a large heatproof bowl to stop the browning process.
Add apple cider to the same empty pan. Take care when pouring the cider into the pan as it may sizzle a bit.
Cook the cider over medium-high heat, stirring occasionally, until the cider has reduced to ½ cup. This process will take about 35 minutes. The best way to measure your reduced cider is to simply pour it into a glass, liquid measuring cup. If you over-reduced the cider, just add a little more cider to equal ½ cup.
Immediately pour the reduced apple cider into the bowl of browned butter and let cool at room temperature for about 30 minutes.
While the butter and cider mixture is cooling, preheat your oven to 350°F. Line a 9x13-inch baking pan with parchment with an overhang on the two long sides - this will allow you to remove the bars from the pan with ease. Spray the interior of the pan with nonstick cooking spray.
In a small bowl, stir together the granulated sugar and 1 teaspoon of the apple pie spice. Set aside.
In a large mixing bowl, whisk together the flour, remaining apple pie spice, kosher salt and baking soda. When the butter and cider are cooled to room temperature, add in the brown sugar and mix until it is fully incorporated and the sugar is mostly dissolved. Add in the eggs and vanilla and whisk until well combined. Use a rubber spatula to stir the flour mixture into the wet ingredients until the batter is smooth and the flour is completely incorporated.
Transfer the batter into the prepared baking pan and smooth into an even layer with an offset spatula or the back of a spoon. Sprinkle evenly with the apple pie spice-sugar mixture and press it in gently so it sticks.
Bake until the top is browned and a toothpick inserted into the center comes out with only a few moist crumbs attached, 35 to 40 minutes. Let cool for at least 1 hour in the pan before slicing.
Store bars in an airtight container at room temperature for up to 3 days or in the refrigerator for up to 5 days.