Butterscotch Pudding Blondies

2 sticks (1 cup) unsalted butter, at room temperature, plus more for the baking dish
1 1/2 cups firmly packed light brown sugar
2 1/4 cups all-purpose flour
One 3.4-ounce package butterscotch instant pudding mix
3/4 teaspoon baking soda
1/2 teaspoon kosher salt
2 large eggs, at room temperature
1 teaspoon pure vanilla extract
3/4 cup roasted salted cashews, coarsely chopped
3/4 cup semisweet chocolate chips
Flaky salt

Directions

Preheat the oven to 375 degrees F. Line a 9-by-13-inch glass baking dish with aluminum foil, leaving a 2-inch overhang. Grease the foil with butter.
Put the brown sugar in a large bowl. Melt the butter in a small skillet or in the microwave in a glass measuring cup. Stir the butter into the sugar until smooth. Cool to room temperature.
Meanwhile, in another medium bowl, whisk the flour, pudding mix, baking soda and kosher salt together.
Beat the eggs and vanilla into the sugar mixture. Stir in the flour mixture a bit at a time and mix until a smooth, thick batter forms. Fold in the cashews and chocolate chips.
Spoon the batter into the prepared dish and spread evenly. Bake until the blondies are light brown around the edges and a toothpick inserted in the center comes out clean, about 20 to 25 minutes.
Remove from the oven and sprinkle with flaky salt. Let cool in pan slightly, about 15 minutes. Using the overhanging aluminum foil, lift the blondies out of the pan and transfer to a cooling rack to cool completely. Remove the foil and cut into 18 blondies.