Add the water, dark brown sugar, Biscoff cookie butter, cinnamon, cardamom, ginger, and white pepper to a small saucepan over medium heat. Whisk continuously as the mixture heats up to fully melt and incorporate the cookie butter before it comes to a simmer.
1 cup water, 1 cup dark brown sugar, ¼ cup Biscoff cookie butter, ½ tsp ground cinnamon, ⅛ tsp ground cardamom, ¼ tsp ground ginger, ⅛ tsp white pepper
Once the mixture is smooth and beginning to bubble at the edges, reduce the heat to low. Simmer gently for 10 to 12 minutes, stirring occasionally, until the syrup has thickened slightly. Do not let it reach a hard boil.
Remove from heat. Stir in the vanilla extract and flaky sea salt. Taste and adjust the salt, you want a noticeable but not overwhelming finish. This is what makes the cookie butter flavor pop.
½ tsp flaky sea salt, 1 tsp pure vanilla extract
Let the syrup cool for 10 minutes, then pour through a fine mesh strainer into a clean glass jar or squeeze bottle to remove any cookie butter sediment. For an extra smooth syrup, strain twice.
Seal and refrigerate for up to 2 weeks. Shake before each use as the syrup can settle. Start with 1½ to 2 tablespoons per drink and adjust to taste.