Preheat oven to 350 degrees F. Butter and flour 6 (1/2-cup) muffin tins.
In a metal bowl set over a pan of simmering water, melt chocolate with butter, whisking until smooth. Remove bowl from heat and whisk in sugar. Whisk in eggs, 1 at a time, and add remaining ingredients, whisking until combined well. Divide batter among prepared tins and bake in middle of oven until a tester comes out clean, 20 to 25 minutes.
Turn cakes out onto a rack and serve warm or at room temperature with accompaniments.