Photograph: Dan Jones
Melt the butter in a pan, stir in the flour and cook for 1 minute, stirring all the time. Whisk in the milk gradually, then add the sugar. Bring to the boil and simmer for 2-3 minutes.
Stir in the cream and rum to taste; warm through. Pour into a warm jug to serve.
Get ahead: make to the end of step 1 a few hours ahead; cover with clingfilm placed directly on the surface of the sauce. Reheat to serve and stir in the cream and rum.