COOKIES

1. In a medium pot, heat the stout over medium heat. Simmer for 5-10 minutes watching closely to ensure that it does not boil over, until it has reduced to half a cup. Set aside to cool. 
2. Place the chocolate in a heatproof bowl set over a pot of simmering water. Stir until melted, taking care not to get any water in the chocolate. Remove the bowl from the pot, and set aside to cool. 
3. In a small bowl, sift together flour, baking powder and salt. Set aside. In the bowl of a stand mixer, or using an electric hand held mixer, beat the butter and brown sugar until pale and fluffy, 3-4 minutes. Add vanilla, egg and egg yolk, and beat well. Add melted dark chocolate and beat until incorporated, scraping the bowl down well.  With the mixer on low, add a third of the flour mixture, alternating with half of the cooled stout and repeat, starting and ending with the flour mixture. Mix on low until well incorporated, being sure to scrape down the sides and bottom of the bowl well. Chill the mixture for approximately an hour, or until firm, stirring every 15 minutes or so to ensure that it cools evenly. Toward the end of the hour, preheat the oven to 350˚f/180˚c and line three baking trays with parchment paper. 
4. Using a cookie scoop (I use a #30 / 1.5 Tbsp size), Scoop out mounds of dough onto the prepared baking tray, leaving about 10cm between each one to allow for spreading. Bake for 10-12 minutes, or until the cookies are set. Cool on the tray for 15-20 minutes before transferring to a cooling rack to cool completely. 

BUTTERCREAM

1. In a medium pan over a low heat, heat the milk until it is almost boiling. Remove the heat, add the coffee, and steep for 20 minutes. Strain through a very fine mesh strainer, and return the infused milk to the pot, topping up to make one cup if necessary. Return to almost boiling.
2. In a large bowl, combine the sugar, cornflour, egg and egg yolk, and whisk to combine. Whisking constantly, pour half of the hot milk mixture into the sugar and egg mixture. Whisk until combined, then return to the pot with the rest of the milk. Whisk continuously until the mixture thickens, then cook for one minute, stirring constantly. Transfer to a bowl, place plastic wrap on the surface, and leave to cool completely, using an ice bath to speed up this process if necessary.
3. In the bowl of a stand mixer fitted with the whisk attachment, whisk the cooled pastry cream mixture until it is smooth and creamy. With the mixture on high, add the butter a chunk at a time, and mix until the buttercream is homogenous and smooth. Swap the mixer to the paddle attachment and beat for 1-2 minutes until creamy and silky. 

ASSEMBLY

1. Using an offset spatula or a piping bag fitted with a round attachment, place a small amount of buttercream on one cookie, and top with another, pressing lightly to stick. Chill, removing from the fridge approx 45 minutes before serving.