1. Prep the Red Heart Cake: In a medium bowl, combine one box of pound cake mix with 2 eggs, ¼ cup softened butter, ⅔ cup water, and 1 teaspoon of red gel food coloring. Mix on low speed for 30 seconds, then on medium for 1-2 minutes until the batter is smooth. Add a little more food coloring if you want a deeper red hue.
2. Bake the Red Cake: Lightly spray a 9x5-inch loaf pan with non-stick spray (like Baker's Joy) or line it with parchment paper. Pour the red batter into the prepared pan. Bake in a preheated oven at 350°F (175°C) for 45 minutes.
3. Cool & Cut Hearts: Remove the baked red cake from the oven and let it cool completely in the refrigerator for 1 hour. Once chilled, gently turn it out onto a clean surface. Using a bread knife, slice the cake into ¾-inch thick pieces. With your 2¼-inch heart cookie cutter, carefully cut a heart from the center of each slice. Set these adorable hearts aside.
4. Prepare the Surprise Layer: Re-spray the clean 9x5-inch loaf pan. Arrange the red heart cut-outs upright, standing them end-to-end in a straight line down the center of the pan. A small scrap piece of cake at one end can help wedge them snugly in place.
5. Mix the Second Cake: In a separate bowl, prepare the second box of pound cake mix with the remaining 2 eggs, ¼ cup softened butter, and ⅔ cup water. Mix until the batter is smooth and lump-free.
6. Assemble the Cake: Carefully pour a small amount of the plain batter into the pan first, creating a thin base around the hearts. Then, gently pour the remaining batter around the sides of the standing hearts and completely over the top. Lightly shake the pan to ensure the batter settles evenly around the hearts. Remove any temporary "wedge" you used.
7. Bake the Final Cake: Bake at 350°F (175°C) for approximately 40 minutes, or until a toothpick inserted into the side comes out clean. Keep an eye on it, as oven times can vary!
8. Cool & Glaze: Remove the cake from the oven and let it cool completely on a wire rack. While it cools, prepare the simple glaze: whisk together 1 cup of powdered sugar with 1-2 tablespoons of milk, adding the milk gradually until you achieve a thick, pourable consistency.
9. Decorate & Serve: Once the cake is fully cooled, drizzle the homemade glaze generously over the top. Scatter your festive Valentine's sprinkles for a celebratory touch. Allow the glaze to set before slicing into the cake to reveal the delightful heart surprise inside!