To Make The Cake Base:
Preheat your oven to 180°C / 356°F.
In a bowl, combine flour and cocoa powder. Set aside.
Beat eggs in an electric mixer, gradually adding sugar until the mixture is thick, pale, and foamy (about 10 minutes).
Gently fold in the flour and cocoa mixture using a spatula.
Pour the batter into a greased 35 x 25 x 8 cm (15 x 10 x 3-inch) pan and bake for 15 minutes. Let it rest for at least 8 hours or overnight.
To Make The Custard Cream:
Reserve 500 ml of the milk and mix with cornstarch and cocoa powder.
In a pot, heat the remaining 1.5 liters of milk with sugar and vanilla until hot. Gradually stir in the cornstarch mixture until the cream thickens.
Remove from heat, cover with plastic wrap to prevent skin from forming, and let cool for 2 hours at room temperature.
To Make The Whipped Cream:
Whisk heavy cream in an electric mixer with sugar and sifted cocoa powder until thick. Refrigerate until needed.
To Assemble The Cake:
Slice the cake in half and place one half back in the pan.
Spread ⅔ of the custard cream on top, place the other half of the cake on top, and gently press.
Spread the remaining custard cream and ⅔ of the whipped cream over the top layer. Smooth it out and refrigerate.
To Make The Glaze:
Melt butter with cocoa powder in a saucepan. Remove from heat and stir in honey. Let it cool for 2 minutes.
Pour the glaze over the cake and refrigerate for 1 hour.
Slice into 20 squares, and pipe the remaining whipped cream on top of each piece. Serve!