Line a baking sheet with aluminum foil or parchment paper.
Place the brown sugar, white sugar, salt, sherry, orange zest, and orange extract into a heavy saucepan, and stir until well combined. Bring to a boil over medium-low heat, stirring constantly; boil for 1 minute. Stir in the butter.
Remove from heat, and stir in the walnut halves to coat with syrup.
Spread the candied walnut halves out onto the prepared baking sheet, and allow to cool thoroughly, at least 1 hour. Break into bite-size chunks to serve.