Line an 8x8-inch square pan with parchment paper. Set aside.
Place walnuts, almonds, and hazelnuts in food processor. Process until finely ground. Add dates; process until almost paste-like. Add cocoa powder, 1/3 cup of the peanut butter, and salt. Process until well combined.
Turn mixture out into prepared baking pan. Use your fingers or the back of a measuring cup to firmly press into pan, working into all edges until smooth. Refrigerate for 1 hour.
Place remaining peanut butter in a small microwave-safe bowl. Heat for 20 seconds, until softened. Spread peanut butter evenly over brownies and sprinkle with cacao nibs. Refrigerate again for 10 minutes. Slice brownies into 12 squares.