Heat the oven to 350°F (175°C). Grease and line a shallow pan or loaf pan with parchment paper, leaving enough overhang to lift the baked red velvet cake out easily.

Prepare the red velvet cake mix with 1 cup water, 1/2 cup vegetable oil, and 3 large eggs, or follow the exact directions printed on the package. Spread the batter in an even layer and bake for 25-30 minutes, until the center springs back lightly and a toothpick comes out clean.

Cool the red velvet cake completely, then refrigerate or freeze it for 20-30 minutes so it becomes firm enough to cut cleanly. Slice the cake into pieces about as thick as the height of the heart cutter.

Use a 2- to 3-inch heart-shaped cookie cutter to cut one heart from the center of each slice. Prepare enough hearts to create a continuous row down the length of the loaf pan, keeping their pointed ends facing the same direction.

Prepare the white or vanilla cake mix with 1 cup water, 1/2 cup vegetable oil, and 3 large eggs according to the package directions. Pour a thin layer of batter into a prepared 9 x 5-inch loaf pan.

Stand the red velvet hearts upright in a straight line down the center of the loaf pan, pressing them tightly together. Keep the hearts centered and make sure the cutter is not taller than the interior height of the pan.

Carefully spoon the remaining white cake batter around and over the hearts. Add batter gently rather than pouring forcefully, since a heavy stream can tilt or float the hidden hearts.

Bake at 350°F (175°C) for approximately 45-60 minutes, until the outer cake is golden and a skewer inserted near the side, rather than through the red center, comes out clean. If the top browns quickly, watch it closely while the center finishes baking.

Let the finished cake cool in the pan for 10-15 minutes, then transfer it to a wire rack. Cool completely for 1-2 hours before frosting or slicing so the loaf sets and the heart pattern remains neat.

Spread vanilla or cream-cheese frosting over the cooled loaf and add red or pink sprinkles if desired. Use a serrated knife to slice crosswise, revealing one hidden red heart in each serving.