With a fluffy vanilla filling sandwiched with soft chocolatey cookies, this American classic is a satisfying treat.
Preheat the oven to 190C/375F/Gas 5. Line two baking trays with greaseproof paper.
For the chocolate cookies, sift together the flour, cocoa powder, baking powder, bicarbonate of soda and salt. Set aside.
Using an electric whisk, beat the butter and sugar together in a bowl until pale and fluffy. Whisk in the egg and vanilla extract until smooth and well combined. Using a wooden spoon, fold in the buttermilk and the coffee.
Add a quarter of the flour mixture and gently fold in until well combined. Repeat with the remaining flour mixture, adding a quarter at a time, until all of the flour has been incorporated and the mixture has formed a dry dough.
Spoon heaped tablespoons of the cookie mixture onto the lined baking trays, leaving at least 4cm/1½in of space between each. Bake for 9-10 minutes, or until the cookies are firm and cooked through. Remove from the oven and cool on wire racks.
For the vanilla filling, beat all of the filling ingredients together with an electric whisk until light and fluffy.
To serve, spoon a generous amount of the vanilla filling onto the flat side of one cookie and top with another cookie. Repeat with all of the remaining cookies and vanilla filling.
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As the final week of heats begins, five more amateur cooks enter the competition.