1. Make the pine nut sponge cake: Heat oven to 350°. Grease a 12" x 16 ½" baking sheet with butter and line with parchment paper. Melt butter in a 10" skillet over medium; cook until color is a deep golden brown, 6–8 minutes, and let cool. Pulse pine nuts and cake flour in a food processor until finely ground. Using an electric hand mixer, beat sugar and egg yolks in a bowl until pale yellow and thick, about 4 minutes. In a separate bowl and using clean beaters, beat whites until stiff peaks form. Fold pine nut flour, then whites and reserved brown butter, into yolk mixture; spread evenly into prepared pan. Bake, rotating once, until golden brown and cooked through, 8–10 minutes; let cool.