Warm the milk to 100–110°F and stir in yeast. Let sit 5–10 minutes until foamy.
In a large bowl, mix melted butter, vanilla, eggs, and brown sugar. Add the yeast mixture and combine.
Add flour and salt, then knead until smooth and elastic (8–10 minutes).
Place dough in a greased bowl, cover, and let rise for 60–90 minutes until doubled.
Roll dough into a 12×18-inch rectangle.
Mix melted butter and Biscoff cookie butter, then spread over the dough.
Combine crushed cookies, brown sugar, and cinnamon; sprinkle evenly.
Cut into 12 strips and roll into spirals.
Place in a greased baking dish and let rise again for 30–45 minutes.
Optional: Pour heavy cream over rolls before baking.
Bake at 350°F (175°C) for 20–25 minutes until golden.
Beat frosting ingredients until smooth and fluffy.
Spread frosting over warm Biscoff cinnamon rolls and serve.