1 cup butter, divided and room temperature
6 tablespoons baking cocoa
1 cup brown sugar
1/2 cup granulated sugar
1 large egg
1 cup sour cream
1 teaspoon vanilla paste
2 1/2 cups all-purpose flour
1/2 teaspoon salt
1 teaspoon baking soda
18 to 24 large marshmallows, cut in half
Preheat oven to 375 degrees. Line baking sheets with parchment paper.
Melt 1/3 cup butter in the microwave. Add chocolate, stir until smooth. Set aside.
Using a stand mixer beat remaining butter and sugars at medium speed until light and fluffy, about 4 to 5 minutes, stopping occasionally to scrape sides of the bowl. Add egg and vanilla paste, beating until combined. Add chocolate mixture and sour cream. Mix until well combined.
In a medium bowl, whisk together flour, baking soda, and salt. Gradually add flour mixture to butter mixture, beating until combined. Using a cookie scoop, drop dough 2 inches apart onto prepared pans.
Bake until almost done 8 to 10 minutes. Place half of the marshmallow on top of each cookie (cut edge down). Return to oven and cook for exactly 2 minutes and NO LONGER! Remove from pan and let cool completely on wire racks.
Frost with chocolate buttercream frosting.