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.
Heat sugar in a medium sized pan over low heat. Stir constantly with a spatula until sugar melts and turn into an amber color. You can wait for it to turn into 'dark' amber, but be careful not to burn it.
Turn the heat up into medium and add the butter. Use a whisk to mix it thoroughly. The mixture will start forming bubbles at this point.
Once the butter has completely melted, pour the cream slowly and continue whisking.
Boil the mixture.
Take the pan off the heat and add the sea salt. Mix and allow to cool down a bit before pouring on the cake.