Pre-heat the oven to 350° F, and grease muffin tins or line them with cupcake liners.
In a large bowl, mix together the butter, sugar, and cinnamon.
Add the eggs to the cinnamon sugar mixture, and mix well.
Next add in the milk and vanilla extract, continuing to mix well.
Finally, add in the Bisquick mix, and mix everything together just until blended.
Fill each of the greased or lined muffin cups about 1/2 to 2/3 full with muffin batter.
Bake at 350° F for about 25 to 30 minutes, or until the muffins are golden brown and a toothpick or cake tester inserted in the center comes out clean.
Allow them to cool in the muffin tin for about 5 minutes; then take a knife, run it around the edge of each muffin, and pop them out onto a wire rack for dipping.
In a small bowl, mix the sugar and cinnamon together.
Dip the warm muffin tops in the melted butter.
Then dip every muffin in the cinnamon sugar mixture, rolling the top around to make sure it's covered with the slightly crispy cinnamon sugar topping.
Place the muffins back on the wire rack to finish cooling, or serve them while warm.