The combination of cocoa powder and dark chocolate is the secret to making brownies that have intense flavor and also a moist and fudgy texture. Hazelnuts are added to the batter for these brownies, and it pays to peel them yourself by toasting in a dry skillet and then, while still warm, rubbing off the papery skins with a kitchen towel. If you buy blanched (peeled) hazelnuts, toasting them briefly in the skillet will help deepen their flavors.