PREHEAT oven to 375F and line a 12-cup muffin pan with paper liners.
STREUSEL: Combine flour and brown sugar in a small bowl. Add butter and mix with hands until crumbly. Set aside.
BATTER: Whisk flour, baking powder, baking soda, cinnamon, ginger, allspice and salt in a large bowl. Beat eggs, oil, buttermilk, both sugars and vanilla in a medium bowl, then stir into flour mixture until just combined. Stir in carrots.
DIVIDE among cups, then sprinkle tops with pepitas and streusel. Bake until a tester inserted into centre of muffin comes out clean, 18 to 20 min. Cool in pan 10 min. Remove and serve warm. Kitchen note: You can also bake this recipe as a carrot cake loaf.