Put the dry ingredients (flour, cocoa, sugar, baking soda and salt) in the bowl, mix: 30 sec. speed. 3 and set aside.
In the empty bowl, combine the eggs, buttermilk, oil, vanilla, red gel dye and vinegar, emulsify: 1 min. speed 4.
Add the mix of dry ingredients set aside and mix: 1 min. speed 4.
Pour the resulting mixture into a baking tray (22 cm diameter) lined with parchment paper or a detacher for baking sheets.
Bake in a preheated oven at 180°C for about 50-50 minutes (always do the toothpick test).
Allow to cool completely and serve sprinkled with icing sugar as desired... Buon appetito!
You can make buttermilk with the thermomix.