Preheat and Prep Your Cake Pans: Start by preheating your oven to 350°F (175°C). Grease two 8-inch round cake pans with butter or non-stick spray, then line the bottoms with parchment paper.

Mix the Dry Ingredients: In a large bowl, whisk together the flour, cocoa powder, sugar, baking powder, baking soda, and salt. Sift the cocoa powder if it's lumpy.

Add the Wet Ingredients: Crack in the eggs, then pour in the milk, oil, and vanilla. Beat with an electric mixer on medium speed for about 2 minutes until smooth and glossy.

Incorporate the Boiling Water: Carefully stir in the boiling water. The batter will thin out dramatically, which is normal. Give it a final stir by hand to ensure everything's combined.

Bake the Cake Layers: Divide the batter evenly between the pans (about 2 1/2 cups each). Bake for 30-35 minutes, or until a toothpick inserted in the center comes out clean. Let them cool in the pans for 10 minutes, then turn out onto wire racks to cool completely.

Prepare the White Chocolate Mousse (If Using): Bloom the gelatin in cold water for 5 minutes until soft. Melt the white chocolate in a microwave-safe bowl in 30-second bursts, stirring until smooth. Heat 1/2 cup of the heavy cream until simmering, then stir in the bloomed gelatin until dissolved. Combine with the melted chocolate, mixing well. Whip the remaining 1 1/2 cups cream with powdered sugar to stiff peaks, then gently fold into the chocolate mixture in three additions. Chill for 30 minutes to set slightly.

Assemble the Cake: Once cooled, level the cake tops with a serrated knife for even stacking. Place one layer on a serving plate or cake board. Spread or pipe a layer of mousse on top, then add the second layer. Use a thin crumb coat of mousse or buttercream around the outside to seal in crumbs, creating a smooth base for the glaze. Chill the assembled cake for at least 1 hour (or overnight) until firm.

Make the Bloody Mirror Glaze: In a saucepan, combine sugar, 2/3 cup water, condensed milk, and bloomed gelatin (bloom it first in 1/4 cup cold water for 5 minutes). Heat over medium, stirring until sugar and gelatin dissolve—about 4-5 minutes. Remove from heat and add the chopped white chocolate, stirring until melted and smooth. Use an immersion blender to blend for 30 seconds, ensuring no air bubbles. Add red gel coloring a little at a time until you get that deep, bloody hue. For a more intense look, add a touch of black. Let it cool to exactly 90-95°F.

Pour the Glaze: With the cake on a rack over a tray (to catch drips), slowly pour the glaze from the center, letting it flow over the edges for that dripping red mirror glaze effect. Rotate the rack for even coverage. Tap gently to pop bubbles. The result should be a seamless, shiny red coating.

Chill and Serve: Refrigerate the glazed cake for 15-20 minutes to set the glaze fully. Then, transfer to a serving plate. Admire that glossy finish—it’s worth every step!