In a small saucepan, whisk together the flour and sugar. Add the milk and set the stove heat to medium. Whisk smooth and continue stirring or whisking frequently as it cooks.
Continue cooking until the sauce begins to bubble and has thickened, about 3 minutes. Add the peanut butter and whisk to melt.
Remove the pan from the heat, add the vanilla and stir to combine. Add butter and stir to melt. Pour the warm sauce over hot waffles or pancakes.