An interesting dilemma.
The crass answer wouldbe to put bowls of ice in there, but a more scientific answer would be to use reflective foil (such as for putting behind radiators)
As Dextrous says, you can insulate the cupboard, but I’m not sure that’ll look very good. The other option, as Dextrous has eluded to is to take care what you put in the ‘hot’ cupboard. Tins shouldn't be a problem getting warm, neither would dried food like rice and pasta.
Maybe putting a shade on the window would be the better answer. A Roman blind can be pulled down a bit, just to cast shade across the cupboard and lifted as the sun moves.
Another way of keeping the temperature down would be to drill ventilation holes in the top of the cupboard, but I'm not sure I'd like the look of those either
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