In a large bowl, combine the milk, 1/2 cup melted butter, flour, yeast, eggs, agave and salt and whisk until smooth. Cover with plastic wrap and let stand until the batter is very puffy, about 2 hours at room temperature (or refrigerate overnight).
Preheat the oven to 250°. Preheat a waffle iron and brush it with melted butter. Gently stir the batter to deflate it. For each batch, fill the waffle iron about two-thirds full (the batter will spread and rise); brush the waffle iron with melted butter as needed.
Cook the waffles until golden and crisp. Transfer the waffles immediately from the iron to plates, or keep them warm in the oven. Serve the waffles with syrup, fresh fruit and softened butter.