Sieve the dry ingredients flour, cocoa powder, baking soda, salt together for 5-6 times and keep it aside.
Grease the pan with butter and dust with flour and keep it ready.
Take a bowl, add milk, oil, sugar, vineger, vanilla essence and mix till sugar is dissolved and keep it aside for 5 minutes
Now add the dry ingredients and mix with spatula till everything is combined.
Now add the beetroot puree and mix. Use beetroot puree as required to get bright red colour.
Taste the batter and make sure there is no beetroot taste. You can adjust cocoa powder if beetroot taste is there.
Preheat the oven at 160°C for ten minutes.
Bake in the same temperature for 35-40 minutes till the toothpick inserted came out clean.
Cool the cake completely in wire rack.