To prepare the pumpkin custard: in a mixing bowl, blend together all of the ingredients until fully combined and smooth. Place in the fridge until ready to use.

Prepare your streusel by combining the flour, cinnamon, nutmeg, ginger and sugar together in a mixing bowl.

Add the butter, water and nuts and cut into the flour to create a coarse, crumbly texture (either by using a pastry blender or by pinching together with your fingers). Store in the fridge until ready to use.

Preheat your oven to 350°F (177°C). Lightly spray a 9x13" baking dish with baking spray or grease and lightly flour.

In your mixer's bowl, cream together the butter and sugar for 7 minutes on medium speed (or setting 3) or until very light and fluffy. Scrape down your bowl three or four times during the mixing time.

Add the eggs, one at a time, mixing well after each addition to ensure the egg is incorporated fully. Scrape down the bowl between each addition of egg. This stage should take 4 minutes and the batter should have the consistency of mousse.

While your batter is mixing, sift together your dry ingredients, twice, into a medium bowl.

In a separate bowl, combine the buttermilk and vanilla extract.

When you have finished adding your eggs, scrape down your mixing bowl once again and add the flour in three equal measurements, alternating with the milk in equal measurements. I.e.: flour-milk-flour-milk-flour. This stage should mix no longer than 5 minutes.

Scrape down your bowl, and stir one final time by hand, no more than three full circles around the bowl.

Remove the pumpkin puree and streusel from the fridge.

Pour the batter into your prepared pan and create three wells down the length of batter- like trenches the length of the pan- one in the middle and about 2" from the middle one. 1" from each long side of the pan.

Scoop or pour the pumpkin puree into the trenches and give the pan a few taps on to the counter to release any air pockets.

Liberally sprinkle the streusel over the "unpumpkined" sections of the cake and bake for 35-40 minutes or until a toothpick inserted into the cake comes out clean.

Allow to cool for 20 minutes before cutting and serving. Allow to cool for 1 hour if turning out whole to freeze.