Step 1: Preheat your oven to 350°F (175°C). Lightly grease and flour a 9×13 inch baking pan.
Step 2: In a medium bowl, whisk together the all-purpose flour, baking powder, and salt. Set aside.
Step 3: In a large bowl, beat the egg yolks with 1 cup of granulated sugar until pale yellow and thick, about 3-4 minutes. Stir in 1 teaspoon of vanilla extract.
Step 4: In a separate, clean large bowl, beat the egg whites on medium-high speed until soft peaks form. Gradually add the remaining ¼ cup of granulated sugar, continuing to beat until stiff, glossy peaks form.
Step 5: Gently fold the beaten egg whites into the egg yolk mixture in two additions, being careful not to deflate the air.
Step 6: Sift the dry ingredients (flour mixture) over the egg mixture in three additions, folding gently after each addition until just combined. Do not overmix.
Step 7: Pour the batter evenly into the prepared 9×13 inch pan.
Step 8: Bake for 25-30 minutes, or until a wooden skewer inserted into the center comes out clean and the top springs back when lightly touched.
Step 9: Remove the cake from the oven and let it cool in the pan on a wire rack for about 10-15 minutes. While still slightly warm, use a fork or skewer to poke holes all over the top of the cake.
Step 10: In a large bowl or pitcher, whisk together the evaporated milk, sweetened condensed milk, and ½ cup whole milk until well combined.
Step 11: Slowly and evenly pour the milk mixture over the poked cake, allowing it to absorb gradually.
Step 12: Cover the pan loosely with plastic wrap and refrigerate for at least 4 hours, or preferably overnight, to allow the cake to fully absorb the milks.
Step 13: Just before serving, prepare the whipped cream topping. In a very cold large bowl, combine 2 cups heavy cream, ¼ cup powdered sugar, and ½ teaspoon vanilla extract.
Step 14: Beat with an electric mixer on medium-high speed until stiff peaks form.
Step 15: Spread the whipped cream evenly over the top of the chilled Tres Leches Cake.
Step 16: Garnish with fresh berries or a sprinkle of cinnamon, if desired. Slice and serve cold.