First, there were chocolate lava cakes: jumbo-muffin sized chocolate cakes with molten fudge centers. Now, enjoy the sequel: molten-center minis, whose soft core comes not from unbaked batter but from a chunk of solid chocolate added to each tiny cake before baking. Cakes bake, centers melt, chocolate lovers rejoice!
Preheat the oven to 400°F. Grease a 20- or 24-cup mini muffin pan.
Melt the butter, and stir in the cocoa, espresso powder, and hot water.
In a separate bowl, whisk together the flour, sugar, soda, and salt.
Pour the cocoa mixture over the dry ingredients, stirring to blend.
Beat in the buttermilk, yogurt, or sour cream; the eggs, and the vanilla.
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).
Submerge 3 or 4 disks 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.
Bake the cakes for 10 minutes.
Remove them from the oven, and serve them upside-down immediately. (The chocolate sinks to the bottom of the cake slightly while baking; serving them upside-down brings the molten chocolate to the top, so it can melt over the rest of the cake below.)
Repeat with the remaining batter. Cakes freeze beautifully. Reheat briefly in the microwave to melt their centers, just prior to serving.
