01 -  Preheat oven to 400°F (200°C). Line a 12-cup muffin tin with paper liners or grease thoroughly to prevent sticking. 
 02 -  In a large mixing bowl, whisk together all-purpose flour, granulated sugar, baking powder, baking soda, fine sea salt, and ground cinnamon until fully incorporated. 
 03 -  Add cold cubed unsalted butter to the dry ingredients. Using a pastry blender or fingertips, work the butter into the mixture until it resembles coarse crumbs with small, visible pieces of butter. 
 04 -  In a separate bowl, whisk together cooked mashed sweet potato, pure maple syrup, bourbon, large egg, and whole milk until the mixture is smooth and evenly blended. 
 05 -  Pour the wet ingredients over the dry mixture. Using a spatula, gently fold together until just combined, avoiding overmixing. The dough will be thick and slightly sticky. 
 06 -  Divide the dough evenly among the muffin cups, filling each approximately two-thirds full for an even rise. 
 07 -  Combine granulated sugar and ground cinnamon in a small bowl. Using a pastry brush, lightly coat the top of each muffin with melted unsalted butter, then sprinkle generously with cinnamon sugar mixture. 
 08 -  Place tin in preheated oven and bake for 20 to 22 minutes until muffins are golden brown and a toothpick inserted in the center comes out clean. 
 09 -  Allow muffins to cool in the pan for 5 minutes, then transfer to a wire rack to cool slightly before serving warm.