Cinnamon Swirl

1. Preheat the oven to 400F/204C and line a muffin tin with liners or spray with non-stick spray. Set aside.
2. In a small bowl whisk together the brown sugar and cinnamon until fully combined, set aside.

Streusel Topping

1. In a medium bowl whisk together the flour, brown sugar, and cinnamon. Pour in melted butter and use a spatula to mix it all together to moisten everything, this will take a couple of minutes, you want a wet sand texture, thick clumps are good. Set aside while you prepare the cake.

Cake

1. In a large bowl whisk together the flour, baking soda, baking powder, and salt.
2. In the bowl of an electric stand mixer fitted with a paddle attachment or a large bowl with a handheld electric whisk beat the sugar and butter together for 2-3 minutes, scrape down the bowl after the first minute then continue mixing. You want a light fluffy mixture.
3. Add the eggs, one at a time, scraping down the bowl after each addition and making sure the egg is fully combined, there should be no trace of egg yolk. Add the vanilla in with the second egg.
4. Gently fold in 1/3 of the dry ingredients until just almost combined, then gently fold in 1/2 of the sour cream until just combined, then repeat with 1/3 of the dry, the rest of the sour cream, and finish with the remaining flour. Do not over mix.
5. I like to use a medium cookie scoop to add a little bit of muffin batter to the pan, then use a spoon to flatten it down. You want each cavity to be about 1/2 way full. Sprinkle the cinnamon sugar evenly between the 12 muffin cavities. Use a muffin scoop to distribute the rest of the batter, again using a spoon to gently press down into an even layer. Sprinkle the crumble between the 12 muffins and make sure to gently press the crumble into the cake batter.  Bake for 5 minutes then reduce the oven temperature to 350F/180C and bake for an additional 15-20 minutes, insert a toothpick into the center of the cake, moist crumbs are okay but if there is any wet batter the muffins will need to cook longer.
6. Let the muffins cool on a wire rack for at least 20 minutes before attempting to remove from the pan. Best served the day they'are made but leftovers will last in an airtight container for up to 3 days.