Preheat your oven to 350 degrees and spray a muffin tin with non-stick spray.
Make the crumb topping first. To make the topping, melt 1/2 cup unsalted butter in a microwave-safe bowl. Then add 1 1/4 cups flour, 2/3 cup brown sugar, 2/3 cup granulated sugar, 2 tsp ground cinnamon, and a pinch of salt to the bowl.
Mix the topping together with a fork (or with your hands) until it comes together and forms a pebble-like consistency. Set aside.
Now make the muffin batter. In a large bowl, whisk together the flour, granulated sugar, baking soda, salt, and cinnamon.
Then add the buttermilk, melted butter, vanilla, 2 eggs, and 1 egg yolk to the dry ingredients. Stir everything together really well.
Scoop the muffin batter into the prepared muffin tin. Fill up each muffin tin 1/3 to 1/2 full. Do NOT fill the muffin tins all the way!
Top the muffin batter with the crumb topping to the top of each muffin tin cup. There should be enough batter for 14-16 muffins so be sure to save some of the crumb topping for those muffins as well.
Bake the muffins for 15-20 minutes until an inserted toothpick comes out clean. Be sure to not overbake as the topping can burn. Let the muffins cool before removing them from the muffin tin. To remove them, gently wiggle them free of the muffin tin. Enjoy!
Store the muffins in an air-tight container at room temperature for up to one week. After the first day, I heat the muffins in the microwave for 10-15 seconds before enjoying.