Do you mean what we used to call Gypsy Tart ?
If so I remember it well !
Large tin evaporated milk.
12oz Dark muscovado sugar.
10in shortcrust pastry case..prebaked or buy a ready made one .
Pre heat oven to 200C/400F/gas 6.
Whisk the evaporated milk and sugar together
minutes until light and fluffy. The mix should be
coffee coloured. Pour the mix into the pastry case
and bake in the oven for 10 minutes.