To make the langues de chat, beat the butter and icing sugar until pale and smooth. Stir in the almond extract. Whisk the egg white with a fork to break it up, then gradually beat it into the mixture.
Sift the flour with the salt over the top of the mixture, then fold in. Place the mixture into a piping bag fitted with a 9mm round nozzle. Pipe 12 fingers, each 8cm long, onto the baking sheet, leaving a 3.5cm space between each biscuit.
Tap the baking sheet on the underside to release any air bubbles in the dough, then chill for 15 minutes, until the dough is firm to the touch. Heat the oven to 180°C/160°C fan/350°F/Gas 4.
Bake the biscuits for 8–10 minutes, or until golden brown around the edges and pale in the middle. Remove from the oven and leave to cool on the baking sheet for 2 minutes before transferring to a wire rack to cool completely.
Spoon about 50g of the melted white chocolate into a small disposable piping bag and set aside
Gently melt the green cocoa butter in the microwave or by placing it in a bowl stood in a jug of hot water. Add a small amount of green cocoa butter to the remaining melted chocolate to colour it mint green.
Dip 1 cooled langue de chat into the chocolate, lengthways on the diagonal. Allow the excess to run off, then place on a sheet of baking paper to set. Repeat with the remaining biscuits, until all 12 biscuits are half-coated in chocolate.
Snip the end of the piping bag and drizzle the white chocolate in lines from side to side across the langues de chat. Leave to set.