Preheat your oven to 180°C (356°F). In a large mixing bowl or an electric mixer, beat the eggs until frothy. Gradually add the sugar, continuing to beat until the mixture is light and fluffy. Gently fold in the unsweetened cocoa powder and all-purpose flour until just combined.

Pour the batter into a greased 15 x 10-inch baking pan and bake for 15 minutes without opening the door. Let it cool completely for at least 8 hours or ideally overnight.

Mix cornstarch with a portion of milk until smooth. Heat remaining milk with sugar and vanilla extract until hot but not boiling. Whisk in cornstarch mixture until thickened, then let cool completely.

In a chilled mixing bowl, add heavy whipping cream, sugar, and cocoa powder. Whip until stiff peaks form, about 3-5 minutes.

Slice the cake in half horizontally. Spread a layer of custard cream over one half, followed by whipped cream, then place the second half on top.

Melt butter over low heat, whisk in cocoa powder and honey until smooth. Pour warm glaze over the cake and cool in the refrigerator for at least 2 hours.