Line rectangle tin about 28 x 34cm, with nonstick baking paper. Preheat the oven to Gas Mark 3, 160ºC, 325ºF. Place margarine, sugar, golden syrup and black treacle into a pan and heat gently until melted. Leave to cool slightly.
Place the flour and ground ginger into a large bowl and stir. Beat together the eggs, Guinness and milk. Pour the margarine and syrup mixture into the flour with the egg mixture and beat until smooth.
Pour the mixture into the tin and bake for about 35 minutes until well risen. Allow the gingerbread to cool a little in the tin, then turn out on to a wire rack and leave to cool. Make a drizzle by pouring Guinness and syrup into a small pan, bring to the boil and then simmer and reduce for about 10-15 minutes until syrupy.
Spoon drizzle over the cake and cut into about 15 portions. Store in an air tight container.
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