CHOCOLATE SUGAR COOKIES

1. Preheat the oven to 350°f / 180°c. Line three baking sheets with parchment paper. 
2. In the bowl of a stand mixer fitted with the paddle attachment, cream together the butter, granulated sugar, and brown sugar, until light and fluffy, 2-3 minutes. Add the egg and the egg yolk and mix well to combine. Scrape down the edges of the bowl to ensure it is fully incorporated. 
3. In a medium bowl, whisk together the flour, cocoa, salt, baking soda, and coffee or espresso powder. Add to the mixer and mix on low speed until incorporated. Scrape down the bowl and mix again briefly to ensure the mixture is even. 
4. Using a cookie scoop or a spoon, scoop 2 Tbsp (30g) balls of cookie dough, about 8 at a time (see tips). Roll each into a ball using your hands, then roll generously in the sugar and place on the baking sheet, pressing down lightly to form a disc shape. Spread about 8 dough balls evenly on your baking sheet. 
5. Bake the cookies for 12 to 13 minutes, until set around the edges and puffy. Remove from the oven and tap the baking sheet sharply onto the counter, to help the cookies deflate. Use a cookie cutter slightly larger than the cookie to scoot any mis-shaped cookies back into a circle. Allow to cool on the pan for 15 minutes then remove and allow to cool completely on a wire rack. 
6. Repeat the scooping, rolling, and baking process with the remainder of the cookies. 
7. Store cookies in an airtight container until ready to use. 

MARSHMALLOW 

1. In the bowl of a stand mixer fitted with the whisk attachment, combine the first measure of cold water and gelatin. Mix well with a fork, and leave to bloom while you prepare the sugar syrup.
2. Fit a large piping bag with a round piping tip, such as an ateco #407.
3. In a medium pot, combine the water, corn syrup, sugar, and salt. Heat over medium to high heat, stirring occasionally. Heat until the syrup reaches 240˚f / 120˚c, then remove from the heat and leave to cool to 210˚f / 100˚c.
4. Turn the mixer on to medium, and mix for a few seconds to help break up the bloomed gelatin. With the mixer running, VERY carefully pour the hot sugar syrup into the mixer. Turn the speed up to high, and whip for 5-7 minutes, until the marshmallow has doubled in volume, has turned white, and holds somewhat of a peak when you stop the mixer and lift out the whisk.
5. Transfer the marshmallow to the piping bag. Use immediately. 

ASSEMBLY

1. Pair up the sugar cookies into pairs of equal size. 
2. Pipe a mound of marshmallow onto one of the cookies, and leave to sit for 2-3 minutes to allow it to firm up slightly, then top with the second cookie, pressing down lightly to adhere. I like to do one test one first to make sure that the quantity of marshmallow piped onto the cookie is correct and it won't squish out the sides when the second cookie is added. 
3. Repeat with the remaining cookies. 
4. Store in an airtight container at room temperature.