In the microwave or in a metal bowl over a pan of simmering water, melt unsweetened chocolate, 1 cup of the chocolate chips, and the butter stirring occasionally until smooth. Remove from heat and set aside to cool. In a large bowl, whip eggs and sugar until thick and pale, about 2 minutes. Stir in the vanilla and the chocolate mixture until well mixed. Combine the flour, cocoa, baking powder and salt; gradually stir into the chocolate mixture. Fold in remaining 1 cup chocolate chips. Cover dough and chill for at least an hour or overnight.