• Preheat the oven to 425°F/215°C. Spray a muffin tin with nonstick spray or line with 12 muffin liners. Set aside.

Prepare the crumb topping

• In a medium bowl, stir together the flour, brown and granulated sugars, and cinnamon for the crumb topping. Add the melted butter and stir to combine. Break the mixture up with a fork to create a crumbly texture. Set aside. (The mixture will be wet initially, but will dry a bit as you prepare the muffin batter.)

  60 g all-purpose flour

  32 g dark brown sugar

  15 g granulated sugar

  ½ teaspoon ground cinnamon

  56 g unsalted butter

Prepare the cinnamon filling

• In a small bowl, stir together the brown sugar, flour, and cinnamon. Set aside.

  55 g dark brown sugar

  8 g all-purpose flour

  1 teaspoon ground cinnamon

Prepare the muffin batter

• In a large bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.

  240 g all-purpose flour

  9 g baking powder

  ½ teaspoon baking soda

  ½ teaspoon fine sea salt
• In a separate bowl, whisk together the melted butter, sugar, and eggs. Then whisk in the sourdough discard, milk, yogurt, and vanilla and mix until all combined.

  114 g unsalted butter

  200 g granulated sugar

  2 large eggs

  120 g sourdough discard

  30 g whole milk

  60 g plain Greek yogurt

  1 teaspoon pure vanilla extract
• Pour the wet ingredients into the dry ingredients and fold with a silicone spatula or wooden spoon until completely combined. The batter will be thick. If needed, add an additional 1-2 tablespoons.
• Spoon half the batter into the prepared muffin tin, dividing between all 12 cups. Sprinkle the cinnamon filling evenly over each cup of batter, using up all the filling. Add the remaining half of the batter, filling each muffin cup. Use a toothpick or wooden skewer and swirl the muffin batter and cinnamon filling together. Add the crumb topping evenly to the top of each muffin.
• Bake at 425°F/215°C for 5 minutes. Reduce the oven temperature to 350°F/175°C and bake an additional 15 to 18 minutes, until golden brown. Insert a toothpick to the center to check if it's done - it should come out clean.
• Remove the pan from the oven. Allow the muffins to cool in the pan for 5 minutes before removing. Use a knife to loosen the tops from the pan, if necessary. Transfer the muffins to the wire cooling rack to cool an additional 10 minutes before serving. Best served warm!