1. Preheat the oven to 175°C (350°F) and grease a mini muffin pan.
2. Melt bittersweet chocolate and oil together over simmering water, then set aside to cool slightly.
3. In a large bowl, beat granulated sugar, brown sugar, eggs, and vanilla extract on high speed for 5-6 minutes until the mixture doubles in size and becomes glossy.
4. Gradually add the melted chocolate and oil mixture while mixing on low speed to combine.
5. In a separate bowl, sift together flour, Dutch-process cocoa powder, black cocoa powder, espresso powder, and salt.
6. Slowly blend the dry ingredients into the wet mixture until mostly combined, then fold in the chopped bittersweet chocolate evenly.
7. Spoon the batter into the prepared mini muffin cups, filling each about three-quarters full.
8. Bake for 15-18 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.
9. Let the brownie bites cool for a few minutes in the pan before removing. Repeat with remaining batter as needed.