Heat the milk in the microwave for 1 minute. You want the milk to feel fairly hot to the touch but not so hot it would burn your finger.
While milk it heating, add the cookie butter to your hot coffee and mix until it is dissolved (You can use a small whisk, a fork, or a handheld frothing wand).
Then add the brown sugar and vanilla extract and stir again, mixing until dissolved.
Add the hot milk to a mason jar and close the lid. Shake the jar for 1-2 minutes until you see foam begin to form. It is recommended to open the lid after the first 15-20 seconds to allow some of the excess heat to escape.If you have a handheld frothing wand or your coffee machine has a frothing machine you can also use that to froth until it forms a foam. (Some machines have the ability to hot froth so you can skip heating the milk if yours does this.)
Pour the frothed milk into the coffee mixture using a spoon or mason jar lid to stop the foam.
To Garnish
Add whipped cream, melted cookie butter, and garnish with Biscoff cookies.
Biscoff garnish could include crumbled on top of the whipped cream, dipped whole into the whipped cream, or placed around the top edge of the glass with extra melted cookie butter. A caramel drizzle is also nice.
Video
Notes
Cookie Butter Not Melting? Be sure your coffee is hot so that the cookie butter will melt quickly. Also make sure the cookie butter is room temperature.
Using a whisk will break up the cookie butter more quickly.
When using a mason jar to create the froth, you may want to open the lid after the first 15-20 seconds to allow some of the excess heat to escape.