Several variables affect the density of baked goods: the leavening agent used, proper working of the batter, and the type of flour. Although overworking a batter results in a dense crumb, it also results in a flat one thanks to the dissipated carbon dioxide. With that in mind, the ideal way to boost a muffin's density, without sacrificing its airiness, is with high-gluten flour. Substituting high-gluten flour for all-purpose flour in muffin preparations results in a final product that has a dense, yet light, crumb and consistency. (See References 1 and 2)
Preheat the oven to 350 degrees F. Line a muffin tin with 2 1/2-inch-diameter cups with baking liners.
Whisk together 1 1/2 cups high-gluten flour, 1 tsp. baking soda, 1/2 tsp. iodized salt, 3/4 cup white granulated sugar, and 1/4 cup dark cocoa powder in a non-reactive bowl.
Fold in 2/3 cup whole milk, 2/3 cup natural yogurt and 1/4 tsp. vanilla extract until incorporated.
Fill the muffin cups 2/3 with batter and place in the oven.
Bake the muffins for 15 minutes, or until a wood pick inserted in their center comes out clean. Place the muffin tin on a wire rake to cool for five minutes and remove the muffins.
Preheat the oven to 425 degrees F. Line a muffin tin with 2 1/2-inch-diameter baking cups with baking liners.
Whisk together 3 tsp. baking powder, 1/2 tsp. baking soda, 2 tbsp. white granulated sugar and 1 3/4 cup high-gluten flour. (See Reference 3)
Beat one egg in a mixing bowl until frothy, and add 1 cup whole milk and 3 tbsp. melted butter. (See Reference 3)
Fold the egg mixture into the dry ingredients with a rubber spatula or wooden spoon until combined. Scrape down the sides of the bowl and remove the batter from the spoon.
Fill the muffin cups two-thirds full with batter and place in the oven. Bake the muffins for 15 to 20 minutes, or until a toothpick inserted in their center comes out clean. Cool the muffins for five minutes on a cooling rack before removing.