With an electric whisk or a wooden spoon, cream the butter and sugar until light and fluffy. Scrape the bowl’s sides to incorporate all the mixture.
Drop in the eggs, one at a time, beating after each addition. Add the flour and baking powder and beat until smooth. Stir in the ginger and water.
Preheat the oven to 180°C (fan 160°C/350°F/Gas 4). Grease a deep 18cm (7in) square cake tin and line the base with baking parchment.
Place the butter and sugar in a large bowl and cream the mixture as shown below. Add the eggs, then the flour and baking powder. Beat well until smooth and thoroughly combined.
Stir in the stem ginger, and then 175ml (6fl oz) hot water. Pour the mixture into the tin.
Bake in the oven for 45–50 minutes, or until the pudding is well risen, browned on top, and springy to the touch.
About 10 minutes before the pudding is ready, make the toffee sauce: place the butter and sugar in a small pan and heat gently, stirring, until the butter has melted and the sugar dissolved. Stir in the cream and heat gently to warm through.
Cut the pudding into 8 even-sized squares and transfer to serving plates. Spoon over the toffee sauce and serve at once.
Recipe from Mary Berry's Cookery Course, published by DK, £25, www.dk.co.uk.