• Preheat oven to 325°F // 160°C // Gas mark 3. Prepare an 8x8 inch square cake pan by lining with 2 pieces of overlapping parchment and wrapping pan in cake strips, if desired. Secure parchment and cake strips with clips.
• In a medium mixing bowl, whisk together the flour, cornstarch, baking powder, baking soda, cinnamon, pumpkin pie spice and salt. Set aside.
• In a large mixing bowl, beat butter with both sugars until light and fluffy.
• Add eggs, one at a time, beating until fully incorporated, then add vanilla and beat until combined.
• Lastly, add oil and pumpkin purée and beat until smooth.
• Add dry ingredients to butter mixture and beat until smooth.
• Pour cake batter into prepared pan and bake for 25 - 35 minutes. Check cake at 25 minutes, then every few minutes thereafter. Baking time will depend on your oven. Remove cake when a tester comes out with a few moist crumbs attached.
• Let cake cool in pan for 5 minutes before removing from pan and placing on a cooling rack to cool completely.

Make frosting

• Cream butter and cream cheese together with an electric hand mixer until fluffy, about 4 minutes. Add vanilla and a pinch of salt, mix to combine.
• Add powdered sugar in 2 batches, thoroughly mixing between additions and whipping for 30 seconds after second addition to make it light and fluffy.
• Spread frosting over the cake and smooth top with an offset spatula. Top with sprinkles if desired.