Combine raspberries and sugar in a small bowl; mash slightly. Stir until well combined.
Blend cake mix, egg, water, and butter together in a large bowl using an electric mixer on low speed, scraping sides of the bowl. Beat at medium-high speed until smooth, about 3 minutes. Pour 1/2 the batter into a heart-shaped baking pan.
Strain raspberries through a wire-mesh strainer, pushing out as much excess moisture as possible using the back of spoon. Pour strained raspberries over the batter in the pan and spread out to the edges. Pour remaining cake batter on top and spread to cover the entire raspberry layer.
Bake in the preheated oven until a toothpick inserted into the center comes out clean, 37 to 39 minutes. Cool on a wire rack for 5 minutes. Run a table knife around the edges to loosen. Invert carefully onto a serving plate or cooling rack. Let cool completely, at least 20 minutes.
Mix glaze mix and milk together in a microwave-safe bowl until well blended. Heat in the microwave until hot and thickened, about 40 seconds. Add food coloring; mix until smooth and immediately pour warm glaze over cooled cake. Place red milk chocolate pieces on top. Let set for 15 minutes.