Mix dry ingredients: 1¾ cups all-purpose flour, 2 tablespoons cocoa, ½ teaspoon baking soda, and ½ teaspoon salt in a medium-size bowl.
Cream butter and sugar: Use a stand mixer to beat ½ cup butter, softened until smooth. Add in 1 cup sugar and 1 large eggix for two minutes. Add cooled 2 squares unsweetened chocolate, melted and mix. Add 1 large egg and mix for another minute.
Combine liquids: Stir together ¾ cup buttermilk or yogurt, ½ teaspoon instant espresso, optional, and 1 teaspoon vanilla extract.
Finish the dough: Add dry ingredients and buttermilk mixture alternately into the creamed mixture. Mix just until combined. Chill dough for at least 1 hour.
Preheat oven to 400˚F (200˚C). Drop dough by spoonfuls onto a lightly greased cookie sheet. Bake 8–10 minutes, until cookies spring back when lightly touched. Cool completely before frosting.
Make the frosting: Melt 1 cup bittersweet chocolate chips in a microwave on MEDIUM for 1-½ to 2 minutes. Stir and let stand for 3-4 minutes to finish melting. Add ⅓ cup sour cream + another tablespoon. Mix in 1 cup confectioner's sugar until smooth. Add more sour cream if needed for spreadable consistency.
Note: Frosting will be thick and gloppy at first. But as you beat, it will become smooth and shiny. Add another tablespoon or more of sour cream to thin to spreadable consistency. Spread on cooled cookies immediately. Let cookies sit until frosting dries slightly.
Notes
Freeze baked cookies unfrosted. Frosting will hold in the refrigerator for a few days. Microwave for 15 seconds and stir until glossy again.