Preheat the oven to 350 degrees F. Line a 12-cup muffin pan with paper liners.
Add the unsalted butter and semisweet chocolate to a small saucepan. Heat over low heat, stirring frequently until smooth and well combined. Remove from heat and set aside.
Add unsweetened cocoa powder, all-purpose flour, baking soda, baking powder, and fine sea salt to medium mixing bowl. Set aside.
Add eggs, granulated sugar, brown sugar, and vanilla bean paste to large mixing bowl. Whisk until well combined and smooth.
Add the chocolate mixture to the sugar mixture and whisk until smooth.
Fold in 1/2 the flour mixture with spatula. Stir in 1/2 the buttermilk. Repeat with remaining flour mixture and buttermilk. Do not over mix.
Divide batter evenly between cupcake liners.
Bake cupcakes 18 minutes in preheated oven, or until a toothpick inserted in the center comes out clean. Remove from oven. Place on wire rack and cool completely.