Grease a 30 x 20cm cake tin and line with parchment. Heat oven to 180C/ 160C fan/gas 4. Mix the coffee with 2 tbsp boiling water, then set aside to cool.
Cream the butter and sugar together for about 5 mins until light and fluffy, then add the eggs, one at a time, incorporating well after each addition. In a separate bowl, combine the cocoa powder and the flour. Add in the cherry essence and cooled coffee to the butter mixture, followed by the dry ingredients. Mix well, then fold in the cherries.
Spoon into the prepared tin, level the top with the back of a spoon and bake in the centre of the oven for 30-35 mins or until a skewer inserted into the middle comes out clean. Leave to cool completely in the tin.
Once cool, melt the dark chocolate in the microwave or over a pan of simmering water, then spread in an even layer over the top of the brownies.
While the chocolate is setting, mix the butter with the icing sugar and a few drops of green food colouring. Put the buttercream in a piping bag fitted with a writing tip nozzle. Cut the brownies into 20 squares, place two glacé cherry halves on top of each one and use the buttercream to create the cherry stem.