• Pour 4 cups of milk and 2 cups of heavy cream (if using; use 6 cups of milk if you are going to skip the heavy cream) into a heavy-bottomed pot or an instant pot.
• Add ⅓ cup of sugar (if using) and the scraped vanilla seeds and the split vanilla bean pod (if using; or you can use 1 tablespoon of vanilla extract instead) to the pot.
• Set a candy thermometer in the pot, heat the mixture over medium heat, stirring occasionally to prevent scorching, until it reaches about 180°F (82°C). You can taste the mixture to add more sugar if it's needed.
• Remove the pot from the heat and let it cool to around 110°F (43°C). You can speed up this process by placing the pot in an ice water bath.
• Once the milk mixture is cooled to 110°F (43°C), in a small bowl, mix 4 tablespoons of the warm milk mixture with 4 tablespoons of plain yogurt. Stir to mix well.
• Remove the vanilla bean pod from the pot. Pour the yogurt mixture back into the pot, gently stir until well combined.
• Cover the pot with a lid. Keep the mixture at warm temperature of around 110°F (43°C) for 10 -12 hours. You can the proof mode of your oven. If your oven doesn't have this function, wrap the pot in a blanket and put it in the oven with the oven light turned on. Or you can use the "yogurt mode" of your instant pot or yogurt maker.
• After 10 hours, check if the milk has thickened into yogurt. The yogurt should look like pudding, firm enough to uphold a spoon without it sinking to the bottom.
• You can enjoy the yogurt immediately. It will taste better after chilled in the fridge for a couple of hours. If you desire a thicker and creamier texture, follow the steps below to strain it and make Greek yogurt.

Strain to get Greek Yogurt

• Line a strainer with 4 layers of cheesecloth (you can fold a large cheesecloth twice) and set it over a large bowl.
• Pour the yogurt into the strainer, secure the cheesecloth by tying or clipping it, cover the large bowl with plastic food wrap, and allow it to drain in the refrigerator for 4 hours to overnight, or until it achieves your desired thickness.
• Enjoy your homemade thick and creamy Greek yogurt. To preserve any remaining homemade yogurt, transfer it to an airtight container and refrigerate. It can be stored in the refrigerator for up to 1 week. Don't forget to save a few tablespoons for creating an infinite yogurt glitch.