In a medium bowl, stir together flour, cocoa powder, baking soda, baking powder and salt.
In a separate bowl, beat butter and brown sugar with an electric mixer until well-combined.
Beat in egg, vanilla and sour cream until smooth. Stir in flour mixture.
Cover and chill around two hours or until firm enough to shape.
Preheat oven to 350 degrees Fahrenheit.
Shape dough into 1¼-inch balls.
Push a Rolo into the center of each ball.
Bake for 8 minutes and remove from oven; top each with a half of a marshmallow.
Return to oven and bake 1-2 minutes more.
Cool on cookie sheet 2 minutes before removing. Transfer to wire rack to cool completely.
For the Chocolate Drizzle
Combine chocolate with butter in the top of a double boiler, over simmering water.
Stir frequently until melted and smooth. Remove from heat.
Spoon chocolate into a piping bag or just use a spoon and drizzle over the cookies. Allow to cool.
Store in an airtight container for up to 3 days.
Notes
Chill for Easy Handling: Chilling the brownie dough for around two hours before shaping into balls makes it easier to handle and prevents it from spreading too much during baking. This step also allows the flavors to meld and develop, resulting in richer and more flavorful brownie bites.
Careful Marshmallow Placement: When topping each brownie bite with a marshmallow half, ensure that the cut side of the marshmallow is facing down. This helps the marshmallow adhere to the brownie and prevents it from sliding off during baking, ensuring a neat and attractive presentation.
Patience with Chocolate Drizzle: Allow the melted chocolate and butter mixture to cool slightly before drizzling it over the brownie bites. This prevents the chocolate from being too runny, ensuring a controlled and decorative drizzle pattern. If using a piping bag, make sure to cut a small opening to achieve thin and even lines of chocolate.