Preheat the oven to 400°F. Grease a 20- or 24-cup mini muffin pan.
1) Melt the butter, and stir in the cocoa, espresso powder, and hot water.
2) In a separate bowl, whisk together the flour, sugar, soda, and salt.
3) Pour the cocoa mixture over the dry ingredients, stirring to blend.
4) Beat in the buttermilk, yogurt, or sour cream; the eggs, and the vanilla.
5) Pour batter into each of the muffin cups, filling them about 3/4 full. You won't use all of the batter; you'll need to bake this first batch, then bake the remainder (unless you have two mini muffin pans).
6) Submerge 3 or 4 discs of your favorite solid chocolate — about enough to equal the size of a Hershey's Kiss — in the center of each. Note: Hershey's Kisses won't melt as these cakes bake, so pick a chocolate that's meltable, like our Onyx wafers.
7) Bake the cakes for 10 minutes.
8) Remove them from the oven, and serve immediately.
9) Repeat with the remaining batter. Cakes freeze beautifully. Reheat briefly in the microwave to melt their centers, just prior to serving.
Yield: about 32 mini cakes.