Ingredients
Chai Spice Mix

4 tsp ground cinnamon
2 tsp ground ginger
1 tsp ground cardamom
1/2 tsp ground allspice
1/4 tsp ground cloves
Vanilla Chai Cake
1 1/4 Cups (300ml) whole milk
1 chai tea bag
2 3/4 Cups (290g) sifted cake flour* DIY recipe in notes
2 tsp baking powder
1/2 tsp baking soda
4 tsp chai spice mix (recipe above)
1 tsp salt
3/4 Cup (170g) unsalted butter, room temperature
1 2/3 Cups (333g) granulated white sugar
3 large eggs, room temperature
1/2 Cup (120g) sour cream, room temperature
1 Tbsp pure vanilla extract
Vanilla Chai Buttercream
2 Cups (452g) unsalted butter, room temperature
7 Cups (840g) powdered sugar

1 Tbsp chai spice mix (recipe above)
2 tsp pure vanilla extract
2 Tbsp whole milk, room temperature
1/4 tsp salt
Garnish
5 Biscoff cookies, ground into crumbs

Instructions
Make The Chai Spice Mix
In a small bowl, whisk together the cinnamon, ginger, cardamom, allspice, and cloves and set aside.
Make The Vanilla Chai Cake
Place the chai tea bag into a mason jar. In a small saucepan over medium-high heat, warm the whole milk, stirring constantly, until it begins to boil. Pour the boiling milk over the chai tea bag in the mason jar and let steep for 20 minutes before removing the tea bag. Allow the mixture to cool completely to room temperature before moving on.
Preheat the oven to 350°F (170ºC). Prepare three 6-inch or two 8-inch or 9-inch cake pans by spraying the sides with a cooking spray (Baker’s Joy is my favorite) and fitting a wax or parchment paper circle to the bottom of the pan. Alternatively, you can grease and lightly flour the pans.

Sift the cake flour and then measure by spooning and leveling it in your measuring cup. Add the cake flour, baking powder, baking soda, 4 teaspoons of the chai spice mix, and salt into a bowl and whisk to combine. Set aside.
In the bowl of your stand mixer (a handheld mixer works fine too!), cream the butter for on high for two minutes until it’s light and fluffy. Add in sugar and continue to mix on medium-high for another two minutes, scraping down the bowl and paddle as needed. Add the eggs one at a time, then add vanilla and sour cream and mix for one minute on high, scraping down the bowl and paddle once more.

With the mixer on low speed, add in the dry ingredients and mix until just combined. Add the (room temperature!) chai milk mixture slowly and mix until just incorporated. Scrape down the sides and bottom of the bowl and give it a few stirs to make sure there are no lumps (without over-mixing). The batter will be slightly thick, but pourable.
Pour batter evenly into prepared cake pans (no more than 2/3 of the way full) and bake for 30-35 minutes. They’re done when they spring back to the touch and a toothpick inserted into the middle comes out clean. Let the cakes cool in the pan for five minutes before turning them out onto a wire rack for an additional few hours of cooling. Make sure they’re entirely room temperature before applying any frosting.

Make The Vanilla Chai Buttercream
With a hand mixer or paddle attachment on your stand mixer, cream the butter on medium-high until it’s creamy and light (almost white) in color. About 7 minutes. 
 Whisk 1 Tbsp of the Chai Spice Mix into the measured powdered sugar. With the mixer on low, add the powdered sugar mixture a few cups at a time, scraping down after each addition and making sure each addition is fully incorporated before adding the next one. 
Add vanilla, milk, and salt and mix on medium-low for another two minutes until fully incorporated.

Assembly
Once the Vanilla Chai Cakes have cooled completely, fill and frost the layers with Vanilla Chai Buttercream. To create the design pictured, add crushed Biscoff cookies to the sides of the cake by placing them into the palm of your hand and gently pressing them onto the buttercream finish. Then, fit one piping bag with Wilton Tip 4B and another piping bag with Wilton Tip 1M before filling each with Vanilla Chai Buttercream. Pipe rosettes and stars onto the top of the cake in a crescent moon shape, add some cinnamon sticks, and sprinkle more crushed Biscoff cookies on top.