For the streusel: stir flour with icing sugar and baking powder. Add melted butter and vanilla & mix lightly with a fork until moist crumbs form. Set aside with 1/4 cup of pumpkin seeds.
For the muffins: preheat oven to 375°F Spread 1/4 cup of pumpkin seeds on a cookie sheet & toast 5 to 7 minutes - until a few begin to brown & they smell good - watch them closely. Cool them off the cookie sheet.
Grease a 12 muffin tin, including the top (make big muffin topps release easily too).
Sift dry ingredients into a large bowl.
In a second bowl, grate the zest of the orange. Squeeze 1/4 cup orange juice in with the zest. add the egg & beat lightly. Stir in pumpkin puree, buttermilk, juice, vanilla and butter.
Fold wet pumpkin mixture into flour until mostly moistened. Then stir in toasted pumpkin seeds until well mixed; divide batter among cups, heaping slightly in centre (batter completely fills cups). Sprinkle with raw pumpkin seeds; then top liberally with streusel crumbs. Press seeds and crumbs gently into muffin tops.
Bake in center of preheated oven for 25 to 28 minutes or until a toothpick inserted into muffin centre comes out clean and tops are lightly golden.
Rest on cooling rack for 5 minutes; then lift each muffin out of pan onto rack. Cool completely; to serve, lightly sieve a spoonful of confectioner's sugar over muffin tops, if desired.