Melt the butter in a medium saucepan over medium-high heat. Add the grapes and bay leaves and cook, stirring occasionally, until the grapes begin to split, 3 to 5 minutes. Add the cranberries and stir to coat. Add the sugar, grape juice, 1/2 cup water and a pinch of salt.
Bring the mixture to a boil, then reduce the heat to a simmer and cook, stirring occasionally, until the cranberries burst and the sauce thickens, about 20 minutes. (If the berries aren’t bursting after 15 minutes, gently smash with a spatula.)
Discard the bay leaves. Transfer the cranberry sauce to a blender and puree until smooth. Strain through a fine-mesh sieve into a 2-quart dish. Press a piece of plastic wrap directly onto the surface and refrigerate until set, at least 3 hours and up to 1 week.