Preheat your oven to 350°F (175°C).
Grease your 9x9-inch baking pan with butter and line it with parchment paper.
Melt the unsalted butter in a medium saucepan over low heat, then let it cool slightly.
Combine the melted butter with the brown sugar in a mixing bowl and stir until smooth.
Add the large eggs, egg yolk, and vanilla extract to the butter-sugar mixture and whisk until well combined.
Sprinkle the sea salt and all-purpose flour over the wet ingredients and gently fold until just combined.
Fold in the chopped walnuts and white chocolate chips, reserving some for topping.
Pour the batter into the prepared pan, spreading it evenly, and sprinkle the reserved walnuts and white chocolate chips on top.
Bake for 20-25 minutes or until the edges are golden brown and a toothpick comes out clean.
Let the blondies cool in the pan for 10 minutes, then lift them out using the parchment paper and cool completely on a wire rack before slicing.
