Preheat the oven to 350˚F. Line a 12 cavity muffin pan with paper liners and set aside.
In a medium bowl, mix together the flour and sugar. Cut the butter into 1/2 inch pieces and sprinkle it into the mixture. Mix until crumbly. Add in the baking powder, baking soda and nutmeg, and mix until fully combined.
In a large bowl, mix together the sour cream, egg, vanilla extract and pumpkin puree. Slowly mix in the dry ingredients into the wet ingredients and mix until just combined.
Divide the mixture evenly into the prepared muffin pan.
In a small bowl, slowly mix together all of the streusel ingredients until just combined and crumbly. Scoop evenly on top of the muffins.
Bake in the preheated oven for 22-25 minutes, or until a toothpick inserted comes out clean.
Allow to cool in the pan for around 10 minutes before transfering to wire racks to cool completely.