Preheat the oven to 350°F. Line a baking sheet with parchment paper.
In a large bowl, whisk the melted butter, brown sugar, and vanilla until smooth.
Add the flour and salt, then fold in the chocolate chips and walnuts with a wooden spoon until everything is evenly mixed
Spread the dough onto the parchment-lined baking sheet. Pat it into a thin, even layer with your hands (it may not reach the edges). Bake until golden brown all over, about 18 to 20 minutes. Cool completely, then break into pieces.
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Notes
How to store: Let the brittle cool all the way. Any steam trapped in the container will soften the texture. Once cool, store the pieces in a container that is not airtight. They will stay crisp for up to 7 days. Try these add-ins:
Chopped pecans, almonds, pistachios or macadamia nuts instead of walnuts.
Toffee bits for extra caramel notes
Shredded coconut for a hint of chew and tropical taste
Mini chocolate chips for more chocolate in every bite
White chocolate chips mixed with semi-sweet for contrast
A light sprinkle of flaky sea salt on top before baking