In a medium bowl, whisk your flour, baking soda, salt, cinnamon, nutmeg and cornstarch together.
In a bowl fitted with the paddle attachment, beat the butter until it is smooth on medium speed. Add both sugars and beat on medium high speed for 3-4 minutes until it is pale in color and light and fluffy.
Add in your egg, egg yolk and vanilla extract and beat on high speed for 1 minute. Scrape the sides of the bowl, then pour in your biscoff butter and beat on medium speed for 1 minute
Pour the dry ingredients in and mix on low speed until just combined. Fold in your white chocolate chips.
Place your mixing bowl, covering the top in plastic wrap, in the fridge for at least 2 hours.
Preheat your oven to 350°F/175°C and place parchment paper on 2 cookie sheets. Scoop out the cookie dough using a tablespoon or cookie scoop, you want each cookie dough ball to be 2 tablespoons worth of dough. Roll the cookie dough ball in your palms to make it round. Bake the cookies 6 at a time in the middle rack (I have one cookie sheet in the oven at a time) for 10-11 minutes. The cookies should be golden brown.