Cream carefully for best results
In most muffin batters, the butter is melted and combined with the wet ingredients, not unlike pancake batter. The texture of my doughnut muffin is more cakelike than muffinlike (a doughnut-muffin-cake anyone?), and so I begin by creaming the butter with the sugar.
Creaming is a crucial step that too often gets short shrift. It incorporates air into the batter, which is especially important for mixtures such as this one that are too heavy to rely solely on chemical leavens, such as baking powder and baking soda. The sugar cuts into the butter, creating tiny air bubbles that get further expanded during baking by the baking powder and the heat of the oven. Proper creaming, therefore, gives you a nice, light crumb.
Start with your butter at room temperature. Here's where a lot of people go wrong with creaming. Butter that's too cold won't blend with the sugar, and butter that's too warm won't hold the pockets of air. Butter that's the proper temperature is somewhat firm but soft enough to easily poke a finger into.
For best results, use the paddle attachment on a stand mixer and beat on medium, starting with the butter and then adding the sugar in a steady stream.