Preheat the oven to 350°F (175°C). Grease an 8x8-inch baking pan and set it aside.

Melt 1 stick (4 ounces) unsalted butter in a small saucepan over low heat or in a microwave-safe bowl; allow it to cool slightly.

Stir 1 cup packed light brown sugar into the melted butter until smooth.

Add 1 large room-temperature egg and 1 teaspoon vanilla extract; beat or whisk until the mixture is well combined and glossy.

Stir in a pinch of salt and 1 cup all-purpose flour until just combined—do not overmix.

Quickly fold in 1/2 cup dark chocolate chunks, 1/2 cup milk chocolate chunks, and 1/2 cup white chocolate chunks until evenly distributed.

Spread the batter evenly into the prepared 8x8 pan.

Bake for 20–25 minutes, until the edges are set and the center does not jiggle when you gently shake the pan; the blondies should remain moist and slightly gooey.

Cool completely in the pan before cutting into squares and serving.