Cut a rectangle of non stick baking parchment to fit the base and sides of a 30x23x4cm/12x9x1½in traybake tin or roasting tin. Grease the tin and then line with the paper, pushing it neatly into the corners of the tin. Preheat the oven to 160C/325F/Gas 3.
Measure all the ingredients for the traybake into a large bowl and beat well for about two minutes until well blended.
A hand-held electric mixer is best for this but of course you can also mix it by hand with a wooden spoon. (Treacle can be difficult to weigh accurately as it tends to stick to the scale pan. Weighing the treacle on top of the sugar overcomes this problem.)
Turn the mixture into the prepared tin, scraping the sides of the bowl with a plastic spatula to remove all the mixture. Level the top gently with the back of the spatula.
Bake in the preheated oven for about 35-40 minutes, or until the traybake springs back when pressed lightly with a finger in the centre and is beginning to shrink away from the sides of the tin.
Allow the traybake to cool a little, then remove the cake from the tin by easing the paper away from the sides of the tin. Turn onto a cooling rack, remove the lining paper and leave to cool completely.
To make the frosting, mix the icing sugar and syrup together in a small bowl until smooth and a spreading consistency.
Pour the icing over the cake, spread gently to the edges with a small palette knife and sprinkle with the chopped stem ginger to decorate. Allow the icing to set before slicing the traybake to serve.