In a large mixing bowl, combine 1 cup of spelt flour, 1 cup of rye flour, and 1 cup of unfed sourdough starter. Add 2 tablespoons of honey or maple syrup and 2 cups of buttermilk. Stir until well combined. Cover the bowl with a kitchen towel and let it sit at room temperature for about 6 to 8 hours, or overnight, to ferment.
After the sponge has fermented, whisk together 2 large eggs in a separate bowl. Then add 5 tablespoons of melted butter, 1 teaspoon of vanilla extract, and 1 teaspoon of salt. Mix well.
Gently fold the egg mixture into the sponge using a spatula. Be careful not to overmix; a few lumps are perfectly fine. The batter should be thick yet pourable.
Preheat your waffle iron according to the manufacturer's instructions. Lightly grease it with cooking spray or melted butter to prevent sticking.
Once the waffle iron is hot, pour an appropriate amount of batter onto the center of the preheated iron. Close the lid and cook according to your waffle iron’s settings, usually about 3 to 5 minutes, until the waffles are golden brown.
Carefully remove the waffles and serve them immediately with your favorite toppings like fresh fruits, a drizzle of maple syrup, or a dollop of yogurt.