In the bowl of a stand mixer, start beating the aquafaba and cream of tartar on medium speed and increase speed a little after every minute for about 5 minutes until soft peaks form.

Meanwhile, pour sugar, water, and syrup into a medium saucepan and bring to a boil over medium low heat. As it heats, dip a rubber spatula in a cup of water and use to scrape down any sugar that starts to crystallize on the sides of pan. Stir gently until mixture is clear, then cook without stirring until it reaches 240 degrees F on a candy thermometer. Remove from heat and cool for a minute or two.

Turn mixer onto low speed and slowly pour syrup into stiffly beaten aquafaba. Slowly increase the speed every few minutes as it cools for 5 to 10 minutes, then beat on high until fluffy. Add vanilla, salt. cocoa, and coffee, and beat until combined. Scrape into an air tight container and store in fridge until ready to use.

Preheat oven to 325 degrees F (160 C). Beat butter and sugar together in a medium mixing bowl until creamed. Add baking powder, salt, vanilla, and milk and mix until combined. Add flour, cocoa, and coffee, then mix until dough is completely combined.

Turn out onto lightly floured surface and roll into a layer about 1/8 to 1/4 inch thick and cut into small squares or circles, about 1 inch. Place them about 1/4 inch apart on a parchment or silicone lined baking tray and bake for 6 - 8 minutes until edges are browned and crisp. Cool completely.

Spread a small amount of marshmallow cream between 2 cookies and place on tray; repeat with all cookies. Place tray in freezer for 30 minutes to 1 hour until chilled.

Melt Chocolate and oil in a double boiler and remove tray from freezer. Dip cookies into chocolate and place on wire rack to allow excess to drip off. Allow chocolate to dry completely before serving.