Blend butter, sugars, and almond oil/extract to consistency of peanut butter. Add eggs, sour cream, and buttermilk, mix well. Mixture will be very wet. Mix in soda, salt, and cocoa. Mix flour in last in three parts. Chill completed dough overnight.
Using a #30 scoop (I used my Williams Sonoma medium ice cream scoop), scoop onto baking sheet lined with parchment paper. If you don’t have parchment paper you can use a silpat or grease your baking sheet with cooking spray. I was able to fit 6 cookies per standard baking sheet.
Bake cookies for 13 minutes at 350 degrees conventional or 300 degrees convection.*
Prepare the chocolate buttercream frosting ahead or while cookies are cooling. In a mixing bowl combine butter, powder, sugar. Slowly add in milk and vanilla until mixture is smooth. To thin the frosting add more milk, one tablespoon at a time. To thicken the frosting add more powdered sugar, 1/4 cup at a time.
After cookies are cooled, spread with 1-2 tablespoons of frosting. Top with marshmallow cut in half, sticky side down. Melt chocolate chips in a glass bowl in the microwave (stir every 30 seconds- do not overheat or chocolate chips will burn), then pour into a sandwich ziplock bag. Snip the corner of the bag and drizzle melted chocolate over top of cookie with frosting and marshmallow.
*To get the beautiful, perfectly domed cookies that you see at Rubysnap, cook your cookies on convection bake. I do not have a convection oven, so the cookies you see in these photos were baked with a standard oven at 350.