Preheat oven to 180 degrees-C (350 degrees-F). Grease the mini bundt cake tins.
Cream butter and sugar. Add eggs. Mix.
Add vanilla extract and ½ cup of flour.
Add milk and espresso. If mixture is not dry, slowly add the remaining ½ cup of flour.
Add melted chocolate and mix.
Pour mixture on greased bundt cake tins and bake for 20 to 25 minutes.
Let cake cool down and remove from tins.