Heat oven to 325°F.
Grease 12-cup muffin pan or line with baking cups.
Place metal bowl over medium saucepan of barely simmering water (bowl should not touch water).
Add butter and 6 oz chocolate.
Let stand until melted; stir until smooth.
Cool to room temperature.
Whisk in sugar and egg yolks.
Stir in flour and baking powder.
In large bowl, beat egg whites at medium speed until frothy.
Add cream of tartar.
Beat at medium-high speed until soft peaks form.
Fold one-fourth of the whites into chocolate mixture.
Gently fold in remaining whites until mixed.
Spoon batter into muffin cups, filling two-thirds full.
Bake 25 to 30 minutes or until toothpick inserted in center comes out clean.
Cool on wire rack 10 minutes.
Remove from pan; cool completely.
To make chocolate curls for garnish, line 4x2x2-inch mini loaf pan with plastic wrap.
Melt 6 oz of the chocolate in metal bowl set over pan of barely simmering water (do not let bowl touch water).
Pour into mini loaf pan; cool until firm.
Unmold chocolate; remove plastic wrap.
Using vegetable peeler, make curls.
In medium saucepan, combine remaining 6 oz semisweet chocolate and whipping cream.
Heat over low heat until melted and smooth, stirring frequently.
Spread each cupcake with frosting; let cupcakes stand until frosting is cool but not set.
Garnish with chocolate curls.
