In a small bowl, combine 2 1/4 teaspoons instant yeast, 1/3 cup lukewarm whole milk (see baker's notes), and 1 1/2 tablespoons granulated sugar. Gently whisk together and let sit for 20 minutes, or until bubbles appear on the surface.
In the bowl of a freestanding electric mixer, whisk together 2 cups all-purpose flour and 1/4 teaspoon kosher salt. Use a regular ol' drinking glass and twist it around the center of the flour to create a well. Pour the bubbly yeast mixture (from the 1st step) into the center of the well and fit a paddle attachment to the mixer; on low speed, mix until combined.
Turn the mixer to medium speed and add 3 eggs, one at a time, making sure to only add the next egg after the first one has been thoroughly incorporated into the batter. Once all the eggs have been added, add 1 cup melted unsalted butter, 1 teaspoon vanilla extract, and 3/4 teaspoon ground cinnamon. Mix only until the ingredients are combined — the dough will be quite thick and sticky.
Cover the bowl with plastic wrap and let the dough rest in a warm place until it doubles in volume, around 2 hours (less if it's warm). Once the dough has doubled, use a rubber spatula to mix in 1 cup Belgian pearl sugar, and let the dough rest for another 15 minutes.
Once the dough is ready, heat the waffle iron to medium. Pinch 2-inch balls of dough and flatten them slightly. Place the balls of dough in the waffle iron and cook until golden brown. Sprinkle with powdered sugar to serve, and eat immediately.