In a medium bowl, whisk together 3 1/4 cups whole milk, 1/2 cup heavy cream, and 1 tablespoon ground sumac. Use a sharp knife to split a vanilla bean pod lengthwise and scrape the seeds into the mixture. Throw the vanilla bean pod into the mixture as well, and whisk to combine. Cover with plastic wrap and refrigerate for at least 6 hours, preferably overnight, to infuse.
Once the cream has been infused and you're ready to make the creme caramel, preheat the oven to 350 (F).
Place 1 cup of the granulated sugar to a large saute pan or skillet and place over medium heat. Cook for about 5 minutes, resisting the urge to stir but tilt and swirl the pan instead, until the sugar has melted and begins to brown around the edges. You will need to do this slowly and continually until the sugar has turned into a dark amber caramel. Once the sugar has turned into a caramel, immediately pour it into your creme caramel mold and sprinkle 1 teaspoon ground sumac evenly across the surface. Tilt the dish around so that the caramel coats the entire base and rises halfway up the sides of the dish. Place the dish on a wire rack; the caramel needs to set so that it should be firm enough that pouring the custard over the top of it will not disturb it in any way. Place 6 eggs in a large bowl with the remaining 1 cup of granulated sugar, 1 teaspoon pure vanilla extract, and 1/4 teaspoon kosher salt. Whisk until smooth, then strain the infused milk/cream mixture into the eggs through a fine-mesh sieve, pressing the vanilla pod against the mesh to extract as much flavor as you can. Discard the vanilla pod and whisk until the mixture is thoroughly combined but not too frothy.
Pour the mixture into the caramel-coated dish. Carefully transfer the dish to a wide, high-sided baking pan (see baker's notes) and transfer to the preheated oven. Pour hot water from the kettle into the baking pan so that it comes up about halfway up the sides of the creme caramel dish. Bake for about 90 minutes; the creme caramel should still have a wobble in the middle but a skewer inserted into the center of the caramel should come out clean. Don't worry about the wobble! It'll firm up in the fridge. In fact, be more concerned if your caramel doesn't wobble — that means your custard is overcooked.
Once the creme caramel is done, remove the baking pan from the oven and carefully transfer the creme caramel to a wire rack. Cool to room temperature completely, before covering with plastic wrap and transferring to the refrigerator to set overnight.
After the creme caramel has set and chilled, it's ready to be served. If the custard part of the caramel is stuck to the sides of the dish, carefully run a small knife along the edges to release it. Place a large plate (larger than your baking dish) with a rim to catch the caramel on top of the creme caramel, and quickly flip the whole thing over. Gently lift off the dish to release the creme caramel. Enjoy!