Preheat oven to 350 degrees. Line the bottom of a 13x9 baking pan with parchment and coat with cooking spray.
Whisk together flour, baking soda, salt, and spices.
Beat butter and brown and granulated sugars with a mixer on medium-high speed until pale and fluffy. Add eggs and yolk, 1 at a time, beating well after each addition and scraping down sides of bowl as needed. Beat in vanilla and molasses. Reduce speed to low. Gradually add flour mixture, and beat until just combined. Stir in white chocolate.
Spread batter into prepared pan. Bake on the middle rack of the oven until edges are golden and a toothpick inserted in the center comes out clean, about 30-35 minutes. Let cool completely in pan on a wire rack, about 2 hours, before cutting into squares. As the blondies cool, the middle will probably sink in a little more than the sides – don’t worry about it!