1. Preheat the oven to 170°C with heat up and down for 12 minutes. Start greasing five 15-16cm molds with a little butter and cover the bases with greaseproof paper or butter with a little flour to prevent the mixture from sticking.
2. Prepare a buttermilk by mixing the milk with the lemon juice, stir it and let it sit for a few minutes. Meanwhile, in a large, deep container, sift the wheat flour together with the corn flour, the baking powder, the TERANA ground cinnamon and the salt. On the other hand, in another container, beat the rest of the butter and sugar for about three minutes. At the end, you will obtain a smooth and creamy mixture, where it is essential that the sugar grains are not noticeable.
3. Next, beat the egg whites until they reach stiff peaks. Add the buttermilk and vanilla essence to incorporate and then let sit for a couple of minutes. Then, add the flour and egg white mixtures to the container with the butter and sugar. At first, half of the dry ingredients, then all of the egg white mixture and finally, the rest of the dry ingredients. Stir gently after each addition until you obtain a homogeneous dough.
4. Divide the dough into five equal parts, placing each one in a separate container. Add a small amount of gel coloring to each of them: yellow, red, green, blue and purple. Pour each batter into the molds evenly to the point of leaving no lumps. Enter the oven, which should already be at 170°C and cook for half an hour.
5. To check that the mixtures are ready, insert a knife into the dough on one side, take it out and check that there is no smeared mixture. After the time has passed in the oven, wait a few minutes for the molds to cool a little before removing them from the mold and assemble each layer of the cake.
6. Meanwhile, assemble the icing, which must be very cold. When it is very firm, place it in a pastry bag to start the decoration process. Finally, assemble the cake alternating layers of sponge cake and whipped cream, forming a rainbow with the colors. Similarly, cover the outside of the cake. If you want the colored layers to be seen, remove excess cream from the sides. It is recommended to store it in the refrigerator until ready to consume, in order to preserve its texture.