Preheat oven to 350 degrees. Grease 2 12-slot doughnut pans with oil.
In a bowl, beat together the melted butter, sugar, and Biscoff until smooth, about 2-3 minutes. Beat in the eggs, one at a time, then the yogurt and vanilla. The batter might curdle slightly. Add the flour, cinnamon, baking powder, baking soda, and salt. With the mixer running, slowly stream in 1/4 cup of buttermilk. If the batter seems too thick, add 1-2 tablespoons more. The consistency should be slightly thicker than cake batter.
Transfer the batter into a piping bag or a ziptop bag with a corner snipped off. Pipe the batter into the prepared doughnut pans, filling about 2/3 of the way. Bake for 12-15 minutes until the tops are firm and an inserted toothpick emerges with just a few moist crumbs. Cool 5 minutes in the pan, then run a knife around the edges of each doughnut to loose and transfer to a wire rack. Cool completely.
To make the glaze, melt 1/4 cup of Biscoff in the microwave until very smooth, about 35-40 seconds.Cool 5 minutes. Whisk in the powdered sugar and 3 tablespoons of milk. Thin the glaze as needed with additional milk - you want it to be thick with a good drizzle, but not so thick that it clumps.
Melt the additional 2 tablespoons of Biscoff in the microwave for about 30 seconds. This will be for the drizzle.
Dip one side of each doughnut in the glaze and let excess drip off into the bowl. Set on a wire rack with a piece of parchment beneath the rack to catch any stray drips. Choose one of your decorating options for each: cookie crumbles, a whole cookie, or drizzle, or a combination of any. Repeat with the remaining doughnuts.