Butter a 17cm square tin and line it with baking parchment.
Toast the ginger cake slices under a medium grill and chop them roughly once cooled.
Heat sugar, cream, milk, butter, and 50ml boiling water in a deep pan over low heat until dissolved.
Simmer the mixture until it reaches 116°C on a sugar thermometer.
Pour a quarter of the hot mixture into a separate metal bowl.
Beat the ginger and coffee into the remaining mixture in the pan for one minute.
Stir in the cake pieces and beat until the mixture thickens and pulls away from the pan sides.
Pour into the tin, dollop the reserved mixture on top, marble with a skewer, and set before cutting.