Prep the oven and pans: Preheat to 350°F (175°C). Lightly grease your donut pans with nonstick spray or a thin coat of oil.
Combine dry ingredients: In a large bowl, whisk together flour, sugar, crushed cookies, baking powder, cinnamon, and salt until evenly mixed.
Mix the wet ingredients: In a separate bowl, whisk milk, oil, melted cookie butter, egg, and vanilla until smooth and cohesive.
Make the batter: Pour the wet mixture into the dry ingredients. Stir gently with a spatula just until no dry streaks remain. Do not overmix; a few small lumps are fine.
Fill the pans: Transfer the batter to a piping bag or a zip-top bag with a corner snipped off.
Pipe (or spoon) the batter into each cavity, filling them about 2/3 full to prevent overflow.
Bake: Bake for 9–12 minutes, until the tops spring back when lightly pressed and a toothpick comes out clean or with a few moist crumbs.
Cool: Let the donuts rest in the pan for 5 minutes, then gently turn out onto a wire rack. Cool completely before glazing.
Make the glaze: In a medium bowl, whisk cookie butter, powdered sugar, 2 tablespoons milk, vanilla, and a pinch of salt. Add more milk a teaspoon at a time until the glaze is smooth and pourable but not runny. Think thick honey consistency.
Glaze the donuts: Dip the top of each cooled donut into the glaze, let the excess drip off, then place back on the rack.
Sprinkle with crushed Biscoff while the glaze is still tacky.
Let set and serve: Allow 15–20 minutes for the glaze to set. Enjoy warm or at room temperature.