Grease and flour a 12-cup muffin tin or line the cups with paper cupcake liners.
In a mixing bowl, combine 2/3 cup of the diced bacon with the flour, 1/2 cup of brown sugar, baking powder, baking soda, and salt. Stir to blend thoroughly. Set the remaining 1/3 cup of bacon aside for the topping.
In a medium bowl, whisk the milk with the melted butter, 1/2 cup of maple syrup, sour cream, egg, and vanilla extract.
Pour the milk and egg mixture into the dry mixture and stir until blended.
Fill the prepared muffin cups about three-quarters full.
Combine the reserved 1/3 cup of diced bacon, 2 tablespoons of flour, 2 tablespoons of brown sugar, and 1 tablespoon of maple syrup. Stir with a fork or fingers until crumbly.
Sprinkle the crumb mixture evenly over the tops of the muffins.
Bake the muffins in the preheated oven for 17 to 20 minutes, or until a toothpick comes out clean when inserted into the center of a muffin.