Preheat oven to 180 degrees.

Sift together the all-purpose flour, whole wheat flour, baking soda and salt, keep aside in a bowl.

In another bowl beat the eggs, butter and sugar with an electric blender till light and fluffy. Add the buttermilk and vanilla extract to the mixture and whisk.

Add the dry mixture to the batter, 2 tbsps at a time, fold gently to mix. Keep aside.

Take an A4 paper and and draw any pattern of your choice on one half of the paper. The other half should be blank. Place this paper onto a baking tray (9 in × 13 in jellyroll pan) and cover with a greased parchment paper. (You should be able to see the pattern you have drawn through the parchment paper.)

Remove the baking tray from the refrigerator and gently remove the A4 paper from the tray.

Take fresh cream, sugar powder and vanilla extract in a bowl, whip gently till it forms soft peaks. Refrigerate for further use.

Very gently pour the rest of the batter onto the parchment paper, taking care to not disturb the pattern.

Bake for 8-10 minutes. Allow to cool.

Dust a sheet of parchment paper with castor sugar, place on the work surface. Turn the sheet cake onto the paper, remove the parchment paper, trim the edges with a sharp knife.

You now have the sheet cake, half of whose surface is adorned by the pretty pattern you designed.

Cut the sheet cake into half - one half shall have the complete pattern, the other half unadorned.

Spread the cream filling even over the unadorned half. Top it carefully with the other half that is proudly embellished with your pattern. The beautiful pattern facing you in joy !!