01 - Preheat oven to 350°F (175°C). Grease a 9x13-inch baking pan and line with parchment paper for easy removal.
02 - In a sizable mixing bowl, whisk together all-purpose flour, baking powder, baking soda, salt, pumpkin pie spice, granulated sugar, and light brown sugar until thoroughly integrated.
03 - In a separate bowl, vigorously whisk the pumpkin purée, vegetable oil, room temperature eggs, whole milk, and vanilla extract until the mixture is smooth and uniform.
04 - Introduce the wet components to the bowl containing the dry components. Stir gently until just combined; avoid overmixing to maintain cake tenderness.
05 - Pour the batter evenly into the prepared baking pan. Employ a spatula or knife to artfully swirl the batter, creating irregular peaks and valleys for a visually dynamic effect.
06 - Transfer the pan to the preheated oven and bake for approximately 40–45 minutes. Confirm doneness by inserting a toothpick into the center; it should emerge clear.
07 - Permit the cake to cool within the pan for 15 minutes. Subsequently, carefully lift the cake out using the parchment paper and allow it to cool completely on a wire rack.
08 - In a small bowl, combine powdered sugar, 1–2 tablespoons of milk, and cinnamon until a smooth, drizzle-able glaze forms. Generously drizzle the glaze over the completely cooled cake in an abstract, imperfect pattern.
09 - If desired, scatter chopped nuts and a pinch of flaky sea salt over the glazed cake. Slice and serve this delightful creation.