Tres Leches Cake

Preheat oven to 350°. Butter and flour bottom and sides of a glass 13x9" baking dish. Whisk 1½ cups (188 g) all-purpose flour, 1 Tbsp. baking powder, and ¼ tsp. ground cinnamon in a medium bowl to combine.

Beat the whites from 3 large eggs (reserve the yolks) plus 3 large egg whites in a large bowl with an electric mixer until stiff peaks form, 7–8 minutes. Gradually beat in 1½ cups (300 g) sugar. Add 3 large egg yolks one at a time, beating to blend between additions. Beat in 2 tsp. vanilla extract. Add one-third of the flour mixture, followed by ¼ cup whole milk. Repeat with ½ the remaining dry ingredients and the remaining ¼ cup whole milk. Add the rest of the flour mixture and beat just to combine. Pour batter into prepared pan and smooth the top with an offset spatula.

Bake 25 minutes, then reduce heat to 325° and continue baking until cake is golden brown and middle springs back when pressed, 20–25 minutes more. Let cake cool in pan for 15 minutes.

Whisk 1 cup heavy cream, 1 cup evaporated skim milk, 1 cup sweetened condensed milk, 1 Tbsp. dark rum, and ½ tsp. vanilla extract in a medium bowl. Poke holes all over top of cake with a skewer or toothpick. Slowly drizzle half of the milk mixture over cake, letting liquid soak in before adding more. Let cake sit for 10 minutes.

Invert a platter on top of cake. Lift rack and gently invert cake onto platter. Drizzle remaining sauce over. Spoon any liquid collected on sheet over cake. Do ahead: Tres leches cake can be made 2 days ahead; refrigerate, covered, in baking pan.