1.Preheat oven to 180°C/350°F. Grease 20cm x 30cm (8-inch x 12-inch) slice pan; line base with paper, extending paper 5cm (2-inches) above long sides.
2.Drain cherries, reserving syrup for the cherry chocolate ganache. Chop cherries coarsely.
3.Place butter and chocolate in a small saucepan; stir over low heat until melted. Transfer to a large bowl. Stir in sugar and the water, then eggs, sifted flour and amarena cherries. Pour mixture into pan.
4.Bake 30 minutes or until set; cool brownie in pan.
5.Meanwhile, make cherry chocolate ganache. Heat cream and syrup in a small saucepan until almost boiling, add chocolate; whisk until smooth. Refrigerate for 1 hour or until ganache is a spreadable consistency.
6.Turn brownie, top-side up, onto a serving plate. Spread ganache over brownie; cut into 24 pieces. Top each piece with a fresh cherry to serve.
Do-ahead: Brownie can be made a day ahead up to the end of step 4; store, covered, in the fridge. Unlike maraschino and glacé cherries, which are bleached and dyed red, Italian amarena cherries are a small sour cherry preserved in sugar syrup to retain their natural garnet colour and cherry taste. They are available from speciality delis and good greengrocers.