Preheat oven 400 F. Grease and line a Swiss roll pan (rectangular pan) with wax or parchment paper.
Beat eggs, vanilla and sugar until creamy, foamy, and thickening
Gently fold in flour then pour the batter onto prepared pan. Drop food colors all over and spread out batter. Bake for 5-8 minutes.
Turn cake immediately onto a cake rack covered with a slightly damp tea towel.
Peel off the cooking paper and roll nicely. Release roll and spread the sponge lightly with jelly/jam and roll back into a log
Spread chocolate buttercream to form a log. Slice and serve (I love serving it chilled tho!)