For the streusel:
Combine the flour, sugar, oats, cinnamon and ginger.
Add the butter and cut in with a knife or pastry blender until the mixture looks like coarse crumbs.
For the muffins:
Preheat the oven to 350°F (175°C).
Grease and flour or line 18 muffin cups.
Cream the butter and sugar together.
Add the pumpkin puree, buttermilk and molasses.
Beat in the eggs.
Sift the flour, ginger, cinnamon, nutmeg, cloves, baking soda, baking powder and salt into a medium bowl.
Stir through the crystallised ginger.
Add this to the creamed mixture. Blend thoroughly with a wooden spoon but do not over mix.
Scoop into the prepared muffin pans using an ice cream scoop. Sprinkle with the the streusel.
Bake for 20 to 25 minutes or until a toothpick inserted in the centre of a muffin comes out clean. Allow to cool before eating. If you can resist, leave the cooled muffins in a sealed container overnight before serving as it allows the flavours to develop.
Muffins will keep at room temperature for 3 to 4 days, or can be frozen.