To make the pecan crumb topping, sift the flour in a mixing bowl, add the butter cut into cubes, and use your fingertips to rub them together until the mixture resembles breadcrumbs.
Add the light brown sugar, pumpkin spice and chopped pecans, and mix everything together.
To make the sponge, add the soften butter with the dark brown sugar and granulated sugar to a large mixing bowl, and use a hand mixer to beat them together until you have a creamy mixture.
Add the eggs one by one, beaten really well after every addition.
Add the buttermilk, roasted pumpkin puree, pumpkin spice and beat again.
Sift in the flour, add the baking powder, and use a spatula to mix them together until you have a smooth batter.
Preheat the oven to 180 degrees Celsius (350 Fahrenheit).
Butter and flour a rectangle baking tray (30 x 22 x 5 cm / 12 x 9 x 2 inches).
Spread the batter evenly, top it with an even layer of crumb, and bake for 40-45 minutes or until golden and a skewer inserted in the middle comes out clean.
Remove the cake from the oven, and allow it to cool down completely before slicing.