Rainbow Stained Glass Jelly Cake

1. Pink Cotton Chiffon Sponge (Prepare this first so it has plenty of time to cool before assembling the cake) – Preheat oven to 325°F. Line a quarter sheet pan with parchment. Whisk the egg yolks and sugar until slightly thickened. Slowly whisk in the oil, followed by the warm milk and vanilla. Whisk in the red radish powder until completely dissolved and evenly colored. Sift together the cake flour, cornstarch and salt. Fold the dry ingredients into the yolk mixture until smooth. In a clean bowl, whip the egg whites with the cream of tartar until foamy. Gradually add the sugar while whipping. Beat to soft-medium peaks. Fold one-third of the meringue into the yolk batter. Gently fold in the remaining meringue until no streaks remain. Spread evenly into the prepared pan. Bake for 15–18 minutes, or until the cake springs back lightly when touched. Cool completely. Measure the inside of your loaf pan and trim the sponge approximately ⅛ inch (3 mm) smaller on all sides. Set aside.
2. Naturally Colored Stained Glass Gelatin – Bloom the gelatin in the cold grape juice for 10 minutes. Heat the remaining juice with the sugar and salt until steaming. Stir in the bloomed gelatin until fully dissolved. Divide evenly between six bowls. Add the color powders and whisk until smooth. Pour into shallow containers. Refrigerate until completely firm.
3. Vanilla Milk Gelatin – Bloom gelatin in the cold milk for 10 minutes. Heat the milk, cream, sugar and salt until steaming. Stir in the bloomed gelatin until dissolved. Add vanilla. Strain. Allow to cool while preparing the stained glass gelatin.
4. Assembly – Cut the stained glass gelatin into irregular pieces. Place between paper towels for several minutes to remove excess surface moisture. Allow the milk gelatin to cool until it passes the float test.
5. Float Test – Drop one stained glass piece into the milk gelatin. If it sinks immediately, continue cooling. If it hesitates briefly before slowly sinking, the milk gelatin is ready. If it floats completely, gently warm the mixture before proceeding.
6. Fold the stained glass pieces gently into the milk gelatin. Pour into a lightly greased loaf pan. Tap the pan once or twice on the counter to release any large air bubbles. Refrigerate until softly set. The gelatin should be jiggly but no longer sloshy. Carefully place the trimmed sponge on top. Press very gently so the sponge makes full contact with the gelatin without sinking. Cover with parchment. Place an empty loaf pan on top to provide light, even pressure. Refrigerate overnight.
7. To Un-mold – Dip the loaf pan into warm water for 10–15 seconds. Invert onto a serving platter. If needed, dip again for a few additional seconds before trying again.
8. Notes – Drying the stained glass pieces before assembling prevents excess moisture from diluting the milk gelatin and helps them suspend more evenly. The float test is the best indicator that the milk gelatin has cooled enough to suspend the stained glass pieces. Fold gently to preserve the clean edges of the stained glass. Trimming the sponge slightly smaller than the loaf pan creates clean, professional edges after un-molding. Serve well chilled for the cleanest slices.