Make the Cake: Preheat oven to 325 degrees F. Lightly grease a 9-inch springform pan; set aside. In a medium bowl, whisk together the flour, baking powder and salt; set aside.
Using an electric mixer, beat the butter, sugar and vanilla extract on medium speed for 8 to 10 minutes, or until very pale and creamy. Scrape down the sides of the bowl. Add the eggs, one at a time, beating well after each addition. Reduce the mixer speed to low and gradually add the flour mixture, mixing until just combined. Stir the batter with a rubber spatula to ensure all of the ingredients are incorporated.
Transfer the cake batter to the prepared springform pan and baking until a skewer inserted into the center comes out clean, 40 to 50 minutes. Place the pan on a wire rack and allow the cake to cool completely in the pan.
Make the Three-Milk Mixture: Place the cake pan on a rimmed baking sheet lined with wax paper or parchment paper. Using a skewer, poke holes all over the top of the cake. In a large pitcher, whisk together the whole milk, sweetened condensed milk, buttermilk, dulce de leche, and vanilla extract until combined. Very slowly, pour the milk mixture all over the cake. (It will seem like A LOT, but go slowly add use everything.) Refrigerate the cake for at least 3 hours, or overnight, ensuring that all of the milk mixture is absorbed. Remove the cake from the springform pan and place on a serving plate.
Finish with Topping: Place the cream in a large bowl and either by hand or using an electric mixer, whip on medium speed until soft peaks form. Spread the whipped cream on top of the cake, then drizzle and swirl the dulce de leche into the whipped cream. This is best served the day it is assembled, but can be refrigerated for up to 1 more day.
If you can't find dulce de leche at your grocery store, you can buy it online, or use caramel sauce (store bought or homemade).