Preheat oven to 375℉ and line a muffin tin with muffin papers or lightly grease pan.
In a large bowl, whisk together egg, brown sugar, pumpkin puree, buttermilk, vegetable, and vanilla extract until smooth and combined.
Add in all the dry ingredients and stir until just combined.
Evenly fill muffin pan with batter.
In a small bowl, combine the topping ingredients creating a crumble.
Spread the topping evenly over the muffins.
Bake in preheated oven for 16-18 minutes or until just cooked in the middle.
Cool in pan for 2-3 minutes before transferring to a cooling rack.
Serve warm or at room temperature.
Store leftover cooled muffins in an air-tight container for up to three days.