Preheat oven to 425°F. Line a muffin tin with muffin cups. Set aside.
First, make the cinnamon sugar swirl mixture in a microwave safe bowl, microwave the butter, sugar, cinnamon, and salt in 20 second increment, stirring between each. Repeat until the butter is fully melted. Transfer the mixture to a piping bag and set aside.
In a large bowl, combine the brown sugar, eggs, melted butter, sour cream, and vanilla extract.
In a separate bowl, mix together the flour, cinnamon, baking powder, and salt.
Fold the dry ingredients into the wet, mixing only until combined.
Scoop batter into prepared muffin cups, filling just shy of the top of the cups. Snip the tip off the piping bag and make a swirl on top of your muffin or inject each muffin!
Bake for 5 minutes at 425F, then without opening the oven, turn the oven downto 350F and bake for an extra 15 minutes.
