4. Make the vanilla sponge cake: Increase oven to 350°. Grease a 9" x 13" baking dish with butter and line with parchment paper. Whisk almond flour, confectioners' sugar, salt, and vanilla seeds in a bowl until combined; set aside. In a separate bowl and using an electric hand mixer, beat butter and granulated sugar until fluffy, about 3 minutes. Add yolks, one at a time, and mix well after each addition. With the motor running, slowly add dry ingredients into wet until batter forms. In a separate bowl and using clean beaters, beat whites into medium peaks; fold into batter and spread evenly into prepared baking dish. Bake until golden brown, 18–20 minutes. Let cake cool and invert onto a cutting board; peel and discard parchment paper. Using a 2½" round cutter, cut out 12 circles.