Make the waffle batter: Turn the flour and salt in a large mixing bowl. Add 1 tablespoon sugar for savory waffles, or 2 tablespoons sugar for sweet waffles. Whisk to lighten the flour mixture. Crack the eggs into a medium bowl and whisk them lightly. Melt the butter in a medium saucepan over medium-low heat, stirring with a wooden spoon and scraping the browning milk solids on the bottom of the pot to make sure they color evenly, until the butter is the color of a filbert in the shell and the kitchen smells miraculous, about 8 minutes.
Pull the pan off the heat and pour the milk into the butter. Ladle some of the warm milk mixture into the beaten eggs and whisk to warm them. Add another ladleful, whisk, and pour the egg mixture into the milk and butter in the saucepan. Whisk again to combine. Sprinkle the yeast over the surface and let stand for 5 minutes.
Pour the milk mixture into the dry ingredients and whisk lightly. Stir in the vanilla (you may skip the vanilla if you are making savory waffles.) Cover and refrigerate overnight. In the morning, the batter will have risen significantly and be dimpled with tiny fermentation bubbles. Stir the batter down with a ladle. It should be thick, but still distinctly fluid (if the batter appears sluggishly thick, add up to 2 tablespoons additional cold milk one tablespoon at a time, stirring gently after each addition.) Note: the tiny flecks in the batter are milk solids that browned with the butter.
Stir in a cup or so of sweet potato casserole into waffle batter. Cook the waffles: heat the waffle iron on the desired setting. When it's ready, ladle in some batter, and cook until done. The amount of batter you use will depend on the dimensions of your particular waffle iron. Remove and serve immediately with honey butter or another topping. If you're the one on the waffle iron, don't plan on sitting down until the batter is gone. A light dusting of powdered sugar is recommended for visual appeal.