Preheat the oven to 350°F (175°C). Line a 9x13-inch baking pan with parchment paper, leaving an overhang for easy removal, and lightly grease the parchment.

Melt butter

In a small saucepan over low heat or in a microwave-safe bowl, melt 1 cup unsalted butter until just melted. Let cool about 3 minutes.

Mix sugars and wet ingredients

In a large mixing bowl, whisk together 1 cup granulated sugar and 1 cup packed light brown sugar. Add the slightly cooled melted butter and whisk until combined. Add 2 large eggs, 2 tsp vanilla extract, and 1 cup creamy peanut butter. Whisk or beat on medium speed until smooth and homogenous (about 1–2 minutes).

Combine dry ingredients

In a separate bowl, whisk together 2 cups all-purpose flour, 1 tsp baking powder, and 1 tsp kosher salt.

Make batter

Gradually fold the dry ingredients into the wet mixture using a spatula until just combined. Stir in 1 cup mini chocolate chips, reserving a small handful for sprinkling on top if desired. Do not overmix.

Bake

Spread the batter evenly into the prepared 9x13 pan, smoothing the top. Bake in the preheated 350°F oven for 22–28 minutes, or until the center is set and the edges are lightly golden—a toothpick inserted should come out with a few moist crumbs but not wet batter. If you plan to add marshmallow layer and broil, bake for the shorter time to avoid over-browning.

Marshmallow Layer & Topping

Warm marshmallow fluff

When the blondies are nearly finished (about 5 minutes before end of bake), gently warm 1 cup marshmallow fluff with 1 tbsp unsalted butter in a microwave-safe bowl for 10–15 seconds and stir until pourable. Alternatively, heat in a small saucepan over low heat, stirring constantly until soft.

Add marshmallow layer

Remove the baked blondies from the oven and let rest 3 minutes. Spread the warmed marshmallow fluff evenly over the hot blondie surface using an offset spatula. If using 1 cup mini marshmallows for texture, sprinkle them evenly over the fluff now.

Optional broil for color

If you want a toasted marshmallow surface, place the pan under a hot broiler for 20–45 seconds, watching closely until the marshmallow tops are golden brown. Remove immediately to prevent burning. Let cool on a wire rack 20–30 minutes to set the marshmallow layer. For easier slicing, chill in the refrigerator 30 minutes after it cools.

Finish & Serving

Cut blondies

Using the parchment overhang, lift the blondie slab from the pan and place on a cutting board. If chilled, allow 5–10 minutes at room temperature before slicing. Using a sharp knife wiped between cuts, slice into 12 squares (9x13 pan) or desired portions.

Plate and add ice cream

Place a warm (or room-temperature) blondie square on each dessert plate. Top each with a scoop of vanilla ice cream (about 1 scoop per serving).

Garnish

Drizzle about 1–2 tbsp per serving of melted chocolate or chocolate sauce (total 1/4 cup) over the blondie and ice cream. Sprinkle with chopped roasted peanuts (about 2 tbsp per serving from the 1/2 cup) and a light pinch of flaky sea salt if using.

Serve

Serve immediately so the ice cream begins to melt over the warm blondie, creating the signature triple-threat experience: peanut butter blondie, marshmallow fluff, and vanilla ice cream.