1. Bring milk and water to a boil in a large pot and add some salt.
2. Put the corn in the pot and cover with a smaller lid than the top of the pot to keep corn immersed in the boiling liquid.
3. Cook corn for 6 minutes and remove with tongs.
4. In a small bowl, mix together melted butter and sumac and season with salt and pepper.
5. Season corn with salt and pepper.
6. Preheat grill to medium-high heat.
7. Put corn on the grill.
8. Brush corn with the melted butter and cook, turning occasionally, until some of the kernels are golden brown, about 4 minutes total.
9. Serve corn with softened butter and more salt and pepper.