makes ~20 sandwich cookies
Place the flour, cocoa, baking soda, salt, dark chocolate and granulated sugar in the bowl of a food processor. Blitz together until the chocolate has been broken down into a fine crumb texture. Add the vanilla, butter and coffee and blitz again until the dough comes together. Tip out onto a piece of baking paper, pat out into a square, wrap up in the baking paper and chill for at least 2 hours until firm.
Preheat the oven to 180oC (350oF) / 160oF convection.
Roll the dough out to around 5mm (~1/4-inch) thick. Use a 4.5cm (1.75-inch) round cutter to cut out cookies. Transfer to a lined baking sheet spacing the cookies a few cm apart (they'll spread a little when baking).
Bake for 8-10 minutes until firm but still a bit soft in the centre - they'll crisp up as they cool. If you have a patterned cutter you can press this into the cookie now to leave a decorative impression. Leave to cool on a wire rack.
Cream together the butter and icing sugar in a medium bowl until smooth. Stir in the salt and clear vanilla extract (this is what will make the cookies taste more birthday-cakey. If you just have regular vanilla extract that's fine too but it won't taste as 'true'). Gently fold in 1 tbsp of the sprinkles.
Take a cooled cookie, pipe or dollop some of the frosting onto the underside. Sprinkle with some of the multicoloured sprinkles and sandwich together with a second cookie. Repeat with all the cookies.
Store the cookies in an airtight OXO pop container for up to 5 days (They may soften over time as the cookies absorb moisture so are best eaten soon after making).