Preheat oven to 350°F (175°C). In a food processor, grind the Biscoff cookies into fine crumbs and mix with melted vegan butter. Press into a parchment-lined baking dish and bake for 5-8 minutes until golden brown. Let cool.

In the food processor, blend erythritol, cornstarch, and vegan cream cheese until smooth. Reserve 1 cup of mixture for later. Spread the remaining cheesecake batter over the cooled crust.

Combine the reserved cream cheese mixture with pumpkin puree and pumpkin spice in the processor. Blend until smooth. Spread this pumpkin layer over the cheesecake layer.

In the food processor, combine walnuts, brown sugar, gluten-free flour, pumpkin spice, and salt. Add melted vegan butter and water, mixing until streusel clumps together. Sprinkle over the pumpkin layer.

Bake at 350°F (175°C) for 30 minutes. Continue baking for an additional 25 minutes until streusel is golden brown. Cool at room temperature for 30 minutes, then refrigerate for 4-6 hours or overnight.