Preheat the oven to 325°F and line a 9-inch square metal baking pan with parchment paper. I usually drizzle a tiny bit of olive oil or spray into the pan to help it stick to the sides.
In a bowl of a stand mixer, add ⅔ cup butter and 1 cup brown sugar and mix on high speed until light an fluffy (~3 minutes). Be sure to scrape the sides and get everything evenly incorporated.
Add in each of 4 eggs, mixing thoroughly between each addition. Scrape sides of bowl as needed to keep mixture homogenous.
Add in 2 tsp vanilla, 2 jars of Biscoff cookie butter, and ⅓ cup almond flour (or crushed Biscoff cookies). Mix until thoroughly combined.
Add in ⅔ cup flour, 1 tsp salt, and 1 tsp baking powder. Mix until just combined and no streaks remain.
Scrape the batter into the prepared 9"x9" pan and sprinkle with a pinch or two of flaky sea salt. Bake for 60 minutes or until the edges are golden. Optionally, add another sprinkle of flaky sea salt at around the 40 minute mark.
Serve warm (pop into the microwave for 10-20 seconds to reheat) and top with vanilla ice cream, if you so choose! Store in an airtight container for up to a week.