Before you start blending, pop your serving glasses into the freezer for about five minutes. This keeps your milkshake colder for longer once it's poured, and it gives you that frosty, shop-style presentation people love.
Add your vanilla ice cream, pumpkin puree, milk, pumpkin pie spice, brown sugar, and vanilla extract into the blender jar. I like adding the milk first, since it helps everything else blend more smoothly around the blades.
Blend everything on high for about 20 to 30 seconds, or until the mixture looks completely smooth and thick. If it looks too thick to blend evenly, stop and give it a quick stir with a spatula before blending again.
Take a moment to check the texture before pouring. If your shake seems too thick, add a small splash more milk, and if it's too thin, blend in an extra scoop of ice cream.
Pour your milkshake into the chilled glasses right away. Top each one with a swirl of whipped cream and a generous sprinkle of graham cracker crumbs before serving.