Preheat oven to 325 degrees. Prepare an 8 x 8 square pan, and line with parchment paper, leaving enough to fold the paper over the edges on each side. Set pan aside.

First prepare the crumb topping by stirring together the flour, sugars, baking soda and salt. Pour the melted butter over the mixture and mix thoroughly. Mixture will resemble pebbles. Set aside.

In a mixing bowl, combine the flour, sugar, pumpkin pie spice, baking soda, baking powder and salt. Using a hand or stand mixer, mix in the pumpkin, eggs, buttermilk, oil and vanilla. Stop to scrape down the sides of the bowl. Batter should be smooth.

Pour batter evenly into the prepared pan. Top the batter with the crumb mixture breaking apart any big chunks. Start at the edges and work toward the center.

Bake the cake at 325 for 40-45 minutes or until toothpick inserted comes out clean and crumb free. Remove from oven and let the cake cool for 30 minutes before removing from the pan. To remove the cake from the pan, gently pull up on the sides of the parchment paper and lift out of the pan. Dust with powdered sugar.