Preheat your oven to 350°F (175°C).
Line a 9x9 inch baking pan with parchment paper.
Melt the butter in a mixing bowl and allow it to cool slightly.
Add the brown sugar to the melted butter and whisk until well combined.
Add the egg and vanilla extract, whisking until smooth and glossy.
Sprinkle in the salt and flour, gently folding the ingredients together until just combined.
Fold in the dark, milk, and white chocolate chunks.
Pour the batter into the prepared baking pan and smooth the top.
Bake for 20-25 minutes, or until the edges are golden and a toothpick comes out with a few moist crumbs.
Cool in the pan for 10 minutes, then lift out and cool completely on a wire rack before slicing.
