Melt the butter in a medium heavy-bottomed saucepan (not non-stick) over medium heat.
4 tablespoons unsalted butter
Add heavy cream, brown sugar and salt and stir constantly over medium heat until combined. Bring to a boil and let boil for 4 minutes.
1/2 cup heavy cream, 1/2 cup dark brown sugar, 1 teaspoon coarse salt
Remove from heat and stir in vanilla extract until combined. Transfer to a heat-proof container to cool completely at room temperature.
1 teaspoon pure vanilla extract
Once the container is no longer hot to the touch, set aside 1/4 cup (you'll be using this for the blondies), cover the rest and store in the fridge. Use it as a topping for ice cream later!
Blondie bars
Preheat oven to 350°F. Line an 8x8 inch baking dish with parchment paper and set aside.
In a medium bowl, whisk to combine flour, baking powder and salt. Set aside.
1.33 cups all purpose flour, 1/2 teaspoon baking powder, 1/2 teaspoon salt
In the bowl of a stand mixer, combine melted butter and brown sugar and beat on medium speed until combined.
6 tablespoons unsalted butter, 1 cup light brown sugar
Add egg and vanilla and beat to combine.
1 large egg, 1 teaspoon pure vanilla extract
Add dry ingredients to the bowl of wet ingredients and mix on low speed until completely combined. As the batter thickens, you may need to turn it up to a higher speed.
Add chocolate chunks (save a handful for topping) and stir until just combined.
1/2-3/4 cup dark chocolate chunks
In your prepared baking dish, spread batter in an even layer.
Drizzle your prepared butterscotch sauce over the top of the batter and gently swirl with a knife (see notes for more details). Press your reserved chocolate chunks into the top and top with flake salt (if desired).
3-4 tablespoons of your prepared butterscotch sauce, Flake sea salt for topping
Bake for 20 minutes, or until a toothpick inserted into the center (try to aim for a non-butterscotch sauce spot) comes out mostly clean. Let cool in pan for 10 minutes, then left them out of the pan with the parchment paper and transfer to a wire rack to cool completely.
Cut into 12-16 bars, serve and enjoy!
Notes
For the butterscotch sauce swirls:
Drizzle sauce over the batter in 4 evenly spaced parallel lines that run the length of the pan. Turn your pan 90°. Start at the top of one of the rows and with the end of a butter knife, drag it across the lines of sauce, and repeat 4 more times down the length of the pan.