Preheat oven to 350°F (175°C). Line a 9-inch square baking pan with parchment paper. Grease the lining and any exposed parts of the inside of the pan. Alternatively, leave the pan unlined and grease the pan.
Place the butter and unsweetened chocolate in a microwave-safe bowl. Heat in the microwave at half-power in 30-second increments until the butter melts and the chocolate melts when stirred. Stir until smooth. Allow to cool slightly.
Whisk the cocoa powder into the chocolate mixture. Add the sugar and brown sugar, and mix until combined. Mix in the eggs and vanilla.
Add the flour, ground pecans, and salt. Stir just until combined.
Stir in the chopped pecans and chocolate chips.
Transfer the batter to the prepared pan, and spread evenly. Sprinkle the pecan halves and pieces on top of the batter, pressing down lightly. Bake 30-35 minutes, or until a pick inserted into the center comes out with moist crumbs attached.
Cool the brownies completely in the pan on a wire rack. Then use the parchment lining to transfer the brownies to a cutting board and cut into squares.
Notes
Store in an airtight container at room temperature up to 2 days.