Line a greased 9 x 13-inch baking tray with parchment paper. For the cake batter, use a hand mixer to whip the egg whites into a meringue and gradually add in the sugar. Add a drop of red food coloring to the meringue while mixing. Fold in the vegetable oil, flour, and milk, adding only about a third of each ingredient at a time. Pour the cake batter into the baking tray and slam the baking tray down on the countertop a few times to remove any air bubbles from the batter. Transfer the baking tray to a preheated oven and cook at 350°F for 20 minutes.
Remove the baking tray from the oven and let the cake cool for 5 minutes. Flip the cooled cake out of the baking tray and onto the counter and peel off the parchment paper. Cut the cake lengthwise into 3 equal strips. Using a glass as a stencil, cut 2 heart halves out of 1 of the strips.
Place 3 heart-shaped cookie cutters in a greased 10-inch springform pan and arrange the 2 intact cake strips in the pan so that they form a heart.
Place the halved heart pieces of cake in the bottom of the springform pan to form the base of the cake.
For the dark pink raspberry cream, add the heavy cream, sugar, salt, and corn starch to a saucepan over medium heat. Bring the mixture to a boil while stirring constantly, add in the raspberry syrup, and set the mixture aside for later. Repeat the process 2 more times in different saucepans for the medium pink raspberry cream and the light pink raspberry cream. If necessary, add a little bit of food coloring to get the right shades of pink.
Pour the different colors of raspberry cream into the heart-shaped cake one at a time, starting with the darkest shade of pink. To maintain distinct layers, pour the raspberry cream into the pan over the back of a spoon. Transfer the pan to the fridge and chill the cake for 6 hours, or preferably overnight, until the cream layers have completely set.
For the buttercream frosting, whip the softened butter with a hand mixer until light and fluffy and gradually mix in the sweetened condensed milk to create a smooth cream. Transfer the buttercream frosting to a piping bag with a star nozzle. Remove the chilled cake from the springform pan and pipe the buttercream frosting on top of the cake. Decorate the top and base of the cake with fresh raspberries.