Step 3: Prepare the cinnamon swirl mixture by stirring together softened butter, dark brown sugar, and cinnamon. (iheartnaptime.net)
Using a table knife, gently swirl the mixture into the batter to give it a marbled appearance. (foodgal.com)
You might need to swirl the mixture around several times, especially along the cupcake walls (can take a few minutes per chocolate cup). (theloopywhisk.com)