Prep the pan and oven. Heat the oven to 350°F (175°C). Line a 9x13-inch baking pan with parchment, leaving overhang for easy lifting. Lightly grease the parchment.

Melt the butter. In a large microwave-safe bowl, melt 1 cup (2 sticks) unsalted butter. Let it cool for 3–4 minutes so it doesn’t scramble the eggs later.

Whisk in sugars. Add 1 cup packed light brown sugar and 1/2 cup granulated sugar to the melted butter. Whisk until glossy and combined.

Add eggs and vanilla. Whisk in 2 large eggs, one at a time, followed by 2 teaspoons vanilla extract. Mix until smooth and slightly thick.

Stir in peanut butter. Add 3/4 cup creamy peanut butter and whisk until fully incorporated. The batter should be shiny and cohesive.

Combine dry ingredients. In a separate bowl, whisk 2 cups all-purpose flour, 1 teaspoon baking powder, and 3/4 teaspoon kosher salt.

Bring it together. Fold the dry ingredients into the wet using a spatula. Stop when you no longer see dry flour. Avoid overmixing to keep the bars tender.

Add the mix-ins. Fold in 1 cup butterscotch chips and 1 1/2 cups roughly crushed pretzels. Save a small handful of pretzel pieces and chips for the top.

Spread and top. Scrape the thick batter into the prepared pan. Smooth the top. Sprinkle on the reserved pretzels and chips. Add a pinch of flaky salt if you like a sweet-salty hit.

Bake. Bake for 24–28 minutes, until the edges are set and golden and the center is just barely puffed. A toothpick should come out with a few moist crumbs, not wet batter.

Cool completely. Let the blondies cool in the pan on a rack for at least 1 hour. This sets the texture so you get neat, chewy squares.

Slice and serve. Use the parchment to lift the slab out. Cut into 16–24 bars, depending on how generous you want to be.