Fools can be made with any soft fruit; the amount of sugar will vary with the tartness of the fruit.
Melt butter in a large skillet on medium heat, and add berries and sugar. Cover, reduce heat to low, and cook gently, stirring occasionally, until sugar has dissolved and berries have softened, about 5 minutes. Remove pan from heat, and lightly squash fruit with the back of a wooden spoon. Don't mash to a puree; some texture should remain. Cool completely in the refrigerator before proceeding.
Whip cream to soft but not stiff peaks. Gently fold the cream into the cooled fruit mixture, leaving the mixture marbled. Serve immediately in individual bowls.
Gooseberries and black currants are quite tart. Very soft fruits like strawberries, raspberries, and blackberries don't need to be cooked first; just mash with the back of a spoon and combine with only a half cup sugar to get the juices running.