Step 1: Ensure heavy cream and milk are very cold. In a tall container or the frother's jug, combine the cold heavy cream, cold milk, cookie butter, and powdered sugar (if using).
Step 2: Using an electric milk frother (on cold setting), immersion blender, French press, or hand whisk, froth the mixture. If using an electric frother, process for 1-2 minutes until light and airy. For an immersion blender or whisk, blend/whisk for 2-4 minutes, moving the tool up and down to incorporate air, until a thick, pourable foam forms. With a French press, rapidly pump the plunger for 30-60 seconds.
Step 3: The foam should be velvety, airy, and able to hold its shape on top of a drink, but not stiff like whipped cream. If too thick, add a tiny splash more milk and gently re-whisk. If too thin, continue frothing for a bit longer.
Step 4: Prepare your favorite iced coffee, cold brew, or hot coffee in a serving glass.
Step 5: Gently spoon or pour the homemade cookie butter cold foam over the top of your prepared beverage.
Step 6: Garnish with a sprinkle of crushed Biscoff cookies, a dash of cinnamon, or a drizzle of caramel or chocolate sauce for an extra touch of indulgence. Serve immediately and enjoy!