Preheat oven to 400 degrees and grease 8 cups of a standard size muffin pan.
Combine the flour, baking soda, and salt in a large bowl.
Cut the 4 Tbsp cold butter into very small pieces and add to the bowl. Using your hands, work the butter into the flour until the pieces are small and the mixture has a grainy texture.
Stir in the mashed banana and 2 Tbsp of milk and mix until a cohesive dough has formed. If dough seems dry and doesn’t come together well, add more milk in small increments until a workable consistency is reached.
In a large bowl or large ziploc bag, mix together the sugar and cinnamon.
Cut or pinch off tablespoon sized pieces of dough and add to the cinnamon sugar mixture. Mix until pieces are fully coated in cinnamon sugar (if dough is on the stickier side, you may want to do this in batches to keep them from all clumping together).
Mix together the melted butter and brown sugar. Divide mixture evenly between the 8 greased muffin cups. Top with equal amounts of the dough pieces until muffin cups are full. Place muffin pan on a baking sheet (to prevent any errant butter/sugar from dripping onto the floor of the oven), and into the oven.
Bake at 400 degrees until tops are golden brown, about 20-25 min. Remove from oven, run a knife around the edges of each muffin cup and invert monkey bread out onto a plate. I recommend serving warm with a scoop of vanilla ice cream and some pecans if you’re feeling decadent.