Place the semi-sweet dark chocolate in a microwave-safe bowl and heat on medium for 3 minutes. Stir until fully melted and smooth. Add the salted butter and stir until combined.
In a large bowl, combine the melted chocolate mixture with brown sugar, eggs, and all-purpose flour. Mix until smooth.
Fold in the white chocolate pieces and chopped walnuts.
Drop heaping teaspoons of dough onto an ungreased cookie sheet and bake in a preheated 350°F oven for 12 minutes.
Transfer cookies to a wire rack and cool completely before serving.
Notes
These cookies freeze well and can be stored in an airtight container for up to a week. For an extra touch, sprinkle a pinch of sea salt on top before baking.