There are different types of treacle, Black is only one, Golden Syrup is a lighter-coloured type of treacle and often substituted with corn syrup in recipes where black treacle is not available. In Britain, the main producer of black treacle (and Golden Syrup) is sugar refining company Tate and Lyle.
Roasted grapes are sophisticated. Toss grapes with olive oil, salt, and pepper, and roast them on a sheet pan for about 30 minutes at 425Â°F. The grapes will get blistered and caramelized on the outside, and extra juicy and practically jammy on the inside.
Add the corn sugar to the wine about 1 pound at a time, stirring between each addition to dissolve the sugar. Measure your sugar level of the juice (juice only, no pulp), being careful not to go above 1.085. You may need more or less sugar than the recipe calls for based on the ripeness of your grapes.