Step 1 Put the chocolate in a small heatproof bowl.
Step 2 Combine the cream, milk, espresso powder and a pinch of salt in a 2-quart heavy saucepan and bring just to a boil, stirring until the espresso powder is dissolved. Pour over the chocolate, then whisk gently until the chocolate is melted and the mixture is smooth.
Step 3Whisk together the egg yolks, sugar and a pinch of salt in a medium bowl. Add the warm chocolate mixture in a slow stream, whisking constantly. Pour the custard through a fine-mesh sieve into a 1-quart glass measuring cup and let it cool to room temperature (to prevent condensation from diluting the pots de creme when covered), stirring occasionally, about 15 minutes.
Step 4Put a rack in the middle of the oven and heat the oven to 300 degrees. Line the bottom of a baking pan large enough to hold the pot de creme cups with a folded kitchen towel and arrange the cups on the towel.
Step 5Divide the custard among the cups and add enough boiling water to the baking pan to reach halfway up the sides of the cups. (If the cups have lids, do not use during baking.) Cover the pan tightly with foil and poke a few holes in the foil with a skewer. Bake until the custards are set around the edges but tremble slightly in the centers, 30 to 35 minutes.
Step 6With tongs, transfer the cups to a rack to cool completely, uncovered, about 1 hour. Refrigerate, covered, until cold, at least 3 hours. Serve garnished with chocolate curls and chocolate-covered espresso beans, if desired.