• Preheat the oven to 350°F. Lightly grease a 9x13-inch cake pan with nonstick spray or butter.

Make the streusel:

• In a large bowl, combine 3 cups of flour, 1 cup of brown sugar, 1/2 cup of granulated sugar, 1 teaspoon of kosher salt, 1 tablespoon of cinnamon, and 1 tablespoon of pumpkin pie spice.
• Melt 1 cup of butter in a microwave-safe bowl, then pour the melted butter into the dry ingredients. Mix everything together until the texture resembles wet sand with some larger chunks. It's fine if a bit of loose flour remains.
• Set the streusel aside, and if you have space, place it in the fridge.

For the cake:

• In a large bowl or the bowl of a stand mixer, combine the flour, sugar, baking powder, baking soda, salt, pumpkin pie spice, and cinnamon.
• Cut the softened butter into small chunks. Make sure the butter is soft enough to work with but still holds its shape—just leave it out of the fridge for a few minutes. Add the butter chunks to the dry ingredients.
• Open the can of pumpkin puree and scoop out about half of it with a spatula. Add it to the bowl with the flour and butter.
• Using the paddle attachment on a stand mixer or electric beaters, mix the butter and pumpkin into the dry ingredients. As you blend, the butter will start to moisten the flour, and the mixture will come together into a ball. Be sure to scrape down the sides and bottom of the bowl as needed. Stop mixing once the butter is fully incorporated—avoid overmixing. If the dough seems too dry, add a little more pumpkin puree until it comes together.
• In the same bowl where you melted the butter, add the remaining pumpkin from the can. Whisk in 4 eggs, 1/4 cup of vegetable oil, and 1 tablespoon of vanilla. Mix well until everything is smooth.
• Gradually add the egg mixture to the flour mixture in 3 parts, whisking for about 20 seconds after each addition. Once combined, whisk for an extra minute until the batter becomes light and fluffy.
• Spread 2 cups of the batter into the prepared cake pan. Sprinkle 1 cup of streusel evenly over the top, breaking up any large pieces with your hands.
• Spread another 2 cups of batter on top, smoothing it out as much as possible. Add another 1 cup of streusel.
• Spread the remaining batter evenly over the top, followed by another 2 cups of streusel. (You’ll have some leftover streusel—store it in the fridge.)
• Bake the cake at 350°F for 35 minutes.
• Quickly remove the cake from the oven, leaving the oven door closed. Sprinkle the remaining streusel evenly over the top, focusing on areas where the cake has puffed up. Work fast and return the cake to the oven as soon as possible, keeping the door shut.
• Bake for another 10-15 minutes. The total baking time should be around 45-50 minutes, depending on your oven and pan type. To check if it’s done, insert a toothpick into the center—it should come out clean, with no wet batter. The cake should also feel firm, without wobbling when you shake the pan.
• Let the cake cool on a wire rack for 15-20 minutes before slicing.

Make the icing:

• In a medium bowl, beat the softened butter until smooth. You can do this by hand with a whisk if the butter is very soft.
• Add the vanilla, kosher salt (use less if you're using table salt), milk, and powdered sugar. Mix until the icing is very smooth. If it’s too thick, add the milk a teaspoon at a time until you reach the desired consistency.
• Drizzle the icing over the cake. You can either cover the entire cake or drizzle it on individual servings. I like to use about half the icing on the whole cake and save the rest for each serving.
• For the best taste, serve the cake warm! If it has cooled, you can microwave individual servings for 10-20 seconds before serving.
• Store the cake covered on the counter for 2-3 days. After that, keep it in the fridge.