1. Preheat your oven to 350°F (175°C) and grease a 9×13 inch baking pan.
2. In a large mixing bowl, whisk together the flour, sugar, pumpkin pie spice, baking powder, baking soda, and salt.
3. In a separate bowl, combine the pumpkin puree, vegetable oil, milk, eggs, and vanilla extract. Mix until smooth.
4. Gradually add the wet ingredients to the dry ingredients, stirring gently until just combined. Be careful not to overmix.
5. Pour the batter into the prepared baking pan, spreading it evenly.
6. To make the streusel topping, combine flour, brown sugar, and cinnamon in a bowl. Cut in the cold butter with a fork or pastry cutter until the mixture resembles coarse crumbs. Sprinkle this over the cake batter.
7. Sprinkle chopped pecans over the streusel layer if using.
8. Bake in the preheated oven for 35-40 minutes, or until a toothpick inserted into the center comes out clean.
9. Let the cake cool in the pan for about 10 minutes before slicing and serving.

A tip from my mum: always let the cake cool slightly; it allows the flavors to meld beautifully, creating a richer taste experience.