Preheat oven to 400 degrees. Generously butter 4 cups of a standard muffin tin. Sprinkle with granulated sugar, and tap out excess. Set aside.
In the bowl of an electric mixer, cream butter and granulated sugar until light and fluffy. Add eggs, one at a time, beating well after each addition. With mixer on low speed, beat in flour and salt until just combined, scraping down the sides as needed. Fold in chocolate. Divide batter evenly among prepared muffin cups.
Bake just until tops of cakes are set, 8 to 10 minutes. Remove from oven; let stand 10 minutes.
To serve, turn out cakes, and place on serving plates. Dust with confectioners' sugar, and add a dollop of whipped cream, if desired.