Preheat your oven to 400°F (200°C). While the oven heats, grease a 10-inch or 9-inch round baking pan to prevent the pie from sticking. Set the prepared pan aside.
In a large mixing bowl, whisk together the pure pumpkin, milk, vanilla extract, ground cinnamon, baking powder, pumpkin pie spice blend, salt, flour, sweetener, ground flaxseed (or cornstarch, if using), and, if desired, the oil or almond butter for added richness. Make sure all ingredients are thoroughly combined and the batter is smooth. I like to taste the batter at this stage (before baking!) to ensure the spices are just right—feel free to adjust to your preference.
Pour the smoothly mixed batter from Step 2 into the prepared pan, spreading it evenly. Place the pan in the preheated oven and bake for 35 minutes. The pie will still be gooey when you remove it from the oven—this is normal, as it will continue to set as it cools.
Allow the pumpkin pie to cool completely at room temperature in the pan. Once cooled, transfer it, uncovered, to the fridge and let it set for at least 6 hours—overnight is ideal. This patience will reward you with a firm, sliceable crustless pie. I always find the flavor deepens after chilling, so don't skip this step!