Place 70 g unsalted butter in a light-coloured saucepan, and melt over medium low heat. Continue heating while stirring constantly until the milk solids in the butter turn a toasty brown colour. Take off the heat and leave to cool slightly so you don't damage your food processor with piping-hot butter.
Add 220 g speculoos cookies to a food processor and blitz to a fine crumb. Pour in the cooled but still liquid browned butter, and process until smooth. If it's still looking sandy or too thick, add a little melted butter or neutral-flavoured oil to thin it out. Keep in mind it will thicken and solidify in the fridge, so you want it to be quite loose and pourable at this stage—somewhere between heavy cream and sour cream in consistency.
Add ½ teaspoons ground cinnamon and large pinch fine sea salt and process to combine. At this point if you want to make it smoother, you can add it to a blender and blend until smooth. It will never be as smooth as store-bought cookie butter, which is made using industrial equipment and includes stabilizers and emulsifiers, but a good blender will get you pretty close.
Transfer to an airtight container and refrigerate. Once cold, the cookie butter will solidify so you'll need to bring it back to room temperature to spread it or mix it into a cheesecake or other dessert recipe. It should last 2–3 weeks in the fridge, or frozen for up to 3 months.
