Crush the cookies. Add the Biscoff cookies to a food processor or high-speed blender. Pulse until you get fine, sandy crumbs with no big chunks.
Add dry flavors. Sprinkle in the brown sugar, salt, and cinnamon. Pulse a few times to combine.
Taste the crumbs—they should already taste warmly spiced and slightly sweet.
Stream in melted butter. With the machine running, pour in the melted butter. The crumbs will start to clump together slightly, forming a moist sand texture.
Add vanilla and a splash of lemon. Add the vanilla extract and lemon juice. The lemon won’t make it tart; it just brightens the overall flavor.
Blend in oil and milk gradually. Alternate adding oil and evaporated milk, a tablespoon at a time, blending between additions.
Scrape down the sides as needed. Keep going until the mixture turns silky and spreadable.
Adjust texture. For a thick, scoopable spread, stop when it looks like creamy peanut butter. For a looser, drizzle-ready sauce, add an extra tablespoon or two of milk or oil until it flows off a spoon.
Fine-tune the flavor. Taste and adjust salt for balance, cinnamon for warmth, or brown sugar for sweetness.
Blend again for 10–15 seconds to combine.
Let it settle. Transfer to a clean jar. It will thicken slightly as it rests. Give it 30 minutes at room temperature before using for best texture.