Preheat the oven to 350 degrees F.
Heat the butter, 4 ounces of the chocolate chips and the unsweetened chocolate together in a medium heatproof bowl set over simmering water just until the chocolate melts. Remove the bowl from the heat and allow to cool for 15 minutes. In a large bowl, stir (do not beat) together the eggs, coffee powder, vanilla and sugar. Stir the chocolate mixture into the egg mixture and set aside to cool to room temperature.
In a medium bowl, sift together 1/4 cup of the flour, the baking powder and salt and stir it into the chocolate mixture. Toss the remaining 1/2 cup of chocolate chips and one tablespoon of flour together in a medium bowl and stir them into the chocolate mixture. Spoon the mixture into 4 individual cast-iron skillets (see note at left) and place them on a sheet pan. Bake for 22 minutes exactly. Don’t overbake! A toothpick inserted in the center will not come out clean. Serve warm with a scoop of ice cream.