Preheat the oven to 350°F. Grease a muffin tin or use liners.
Melt chocolate and butter in a double boiler over hot (not boiling) water until all of the chocolate is melted and the mixture is smooth. Set aside.
In a stand mixer, beat eggs, sugar, vanilla, and salt at high speed for about 2-3 minutes or until light and creamy. Blend in melted chocolate at low speed, stopping to scrape sides as needed. Add flour just until incorporated.
Distribute the batter into the prepared muffin tins. Bake for 20-25 minutes or until top is puffed and cracked and a toothpick inserted in the center tests moist.
Allow brownies to cool for about 10 minutes before removing from the tin.