Preheat the oven to 350°F (177°C). Line large baking sheets with parchment paper or silicone baking mats. Set aside.

In a large bowl, whisk together the flour, baking soda, baking powder, and salt. Set aside.

In a medium bowl, whisk together the melted butter and brown sugar until no brown sugar lumps remain. Whisk in the egg, then the Biscoff and vanilla until combined. Pour the wet ingredients into the dry ingredients and mix together with a large spoon or silicone spatula. The dough will be very soft and slick. Fold in the oats and 1 cup of the white chocolate chips, reserving the remaining 1/4 cup for later. The cookie dough will be thick and heavy.

Scoop and roll the dough into balls, about 1.5 Tablespoons (35–40g) of dough each, and arrange on the prepared baking sheets. I suggest using a medium cookie scoop to make it easier. If desired, sprinkle each dough ball lightly with flaky sea salt.

Bake the cookies for 11–13 minutes or until the edges are set. The centers will look very soft, but the cookies will continue to set as they cool.

Remove cookies from the oven. Cool the cookies on the baking sheet for 10 minutes before transferring to a cooling rack to cool completely. While the cookies are still warm, press a few extra white chocolate chips into the tops for looks, if desired. You can also add another light sprinkle of sea salt.

Store tightly covered at room temperature for up to 1 week.