Preheat the oven to 350 degrees F. Grease a mini muffin pan with softened butter.

In a small bowl, combine the chocolate and oil and melt over a double boiler, stirring until fully melted. Set aside to cool.

In a large bowl, mix the sugars, eggs, and vanilla. Beat on high speed for about 5-6 minutes until doubled in size.

While mixing on low speed, gradually add the chocolate oil mixture.

Add the flour, cocoa powders, espresso powder, and salt. Mix until mostly combined.

Fold in the chopped chocolate until evenly dispersed.

Spoon the batter into the prepared mini muffin pan, filling each cup about ¾ full.

Bake for 15-18 minutes or until a toothpick comes out with a few moist crumbs.

Let them cool in the pan for a few minutes before transferring. Repeat with remaining batter.