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1. Preheat oven to 350°F (175°C).
2. Line a 9 by 13-inch (33cm) metal baking pan with parchment paper, leaving an overhang.
3. If you must use glass or ceramic, drop the baking temperature by about 15°F (-10°C) and increase the baking time by 5-10 minutes.
4. In a small bowl, whisk together the flour, baking soda, salt and spices.
5. Set aside.
6. In a small saucepan set over medium heat, or in a microwave-safe bowl in the microwave, heat the butter until melted.
7. Add the brown sugar, granulated sugar, and molasses and whisk well until combined.
8. Let cool slightly, then add eggs, yolk, and vanilla, whisking until combined.
9. Stir in flour mixture with a rubber spatula until just combined.
10. Do not overmix.
11. Stir in white chocolate chips.
12. Spread batter evenly into prepared pan and bake until the edges are solidified and the center is very slightly jiggly, about 27 minutes.
13. Sprinkle with flaky sea salt if desired.
14. Let cool completely before cutting into 2-inch (5cm) squares.
15. It is normal for the center to sink slightly while cooling.
16. Store in an airtight container at room temperature for up to 1 week.

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