Mix the chai spice and granulated sugar together, then cream it in the bowl of a stand mixer with the butter, confectioner’s sugar, salt, and vanilla extract for about 1 minute. You don’t want the mix to get light and fluffy, just well combined. Scrape down the sides of the bowl and add the egg yolk, then beat for another minute.

Add in the flour and mix on low speed until it has combined with the wet ingredients to form a dough. Press it into a log shape about 9″ long and 4″ across on a long piece of plastic wrap, then wrap it well and roll it until smooth. Refrigerate the log for at least three hours, or up to 4 days.

To bake the cookies, preheat the oven to 325F and line a large baking sheet with parchment paper. 

Unwrap the cookie dough and use a pastry brush to brush it all around with the egg wash. Generously sprinkle sugar all around it, gently pressing it into the dough. Using a sharp knife, slice the dough into 1/2″ thick slices. Lay them on the parchment lined baking sheet at least 2″ apart, then bake for 18 minutes, or until the edges and bottoms are golden brown.

Remove the cookies to a cooling rack and cool completely before storing. They will keep up to a week in an airtight container.