In a large bowl, whisk together flour, yeast, brown sugar and salt. Set aside.
In a large saucepan combine the milk, oil, and butter. Heat over medium-low heat until the mixture reaches between 120-130°F.
Add the warm milk mixture to the flour mixture. Using a hand mixer on low speed, mix until combined. Add eggs and vanilla then continue mixing on medium speed for 2-3 minutes. Cover batter and refrigerate overnight.
Remove the batter from fridge and allow to sit at room temperature for 30 minutes. Preheat a waffle iron. Stir baking soda into batter.
Measure about a 1/4 cup of batter onto hot waffle iron and close. Cook until steam subsides and waffle is golden and crisp. Depending on the size of your waffle iron, you may need a little more batter or a little less. Number of waffles made will vary by waffle iron.