Preheat the oven to 350°F (176°C). Coat a muffin pan with nonstick spray and flour; set aside.
To make the Batter
In a bowl, whisk together the flour, baking powder, baking soda, salt, nutmeg and allspice; set aside. In a stand mixer, beat the butter and brown sugar until fluffy. Beat in 1 egg at a time and then the pumpkin and flour mixture on low to medium speed. Mix in the buttermilk. Spoon 1/3 cup of batter into each muffin cup.
To make the Crumble
Mix all the ingredients together until a crumbly mixture forms. Divide the crumble over the muffin batter.
Bake for 30 minutes. Allow the muffins to cool in the pan for 10 minutes before transferring to a wire rack to cool completely.
To make the Icing
Make the icing by beating the butter, cream cheese and vanilla extract until smooth. Mix in the powdered sugar and milk. Fill an icing bag with the cream cheese mixture and pipe on top of the muffins.
Notes
Use an ice cream scoop to get the perfect quantity of batter each time.