Prep: Line a 9x9" pan with parchment paper, leaving overhangs outside the pan for easier removal. Secure the extra parchment paper to the pan with binder clips and set aside.
Crush biscuits: Use a food processor (optional) or place the biscoff cookies in a large ziploc bag. Use a rolling pin to whack the cookies thoroughly until they're finely crushed. Transfer the crushed cookies to a medium bowl.
Add butter: Pour melted butter into the crumbs and stir together until it resembles wet sand.
Press into pan: Add half of the crumb mixture to your lined pan. Spread it out into an even layer and then use the flat bottom of a cup to press it down into a flat layer. Place the pan in the freezer while you make the filling. Keep the bowl with the remaining crumb mixture at room temperature or in a warm place for later.
Whip ice cream mixture: In a large mixing bowl, add heavy cream, pumpkin purée, pumpkin pie spice, and vanilla extract. Use an electric hand mixer to beat the mixture on low speed until it leaves trails and becomes soft peaks.
Fold in condensed milk: Drizzle the condensed milk into the cream mixture while folding it in with a spatula in your other hand until it's all incorporated.
Freeze: Pour the ice cream mixture into the pan on top of the cookie layer. Freeze for at least 2-3 hours until the mixture is no longer jiggly and the surface is firm to the touch.
Press cookie layer on top: Add the remaining crumb mixture on top of the ice cream layer. Spread it out evenly on the surface and then gently press it down again with the bottom of a cup. The ice cream isn't solid throughout at this point so press firmly but not too hard in one spot. The longer you freeze the ice cream layer before adding the top crust, the harder you can press, and the more compact your crust layer will be.
Freeze again: Freeze again for at least 6-8 hours or preferably overnight until solid all the way through.
Slice: Lift the ice cream sandwich block out of the pan by lifting it up with the parchment paper overhangs. If any corners are stuck to the pan, slide an offset spatula around the pan to release. Slice into as many ice cream sandwiches as you'd like before enjoying!