Measure and Sieve the ragi flour, cocoa powder, whole wheat flour, baking soda, and salt. Also, preheat the oven to 180 deg C for 10 minutes.
Take brown sugar in a mixing bowl and add yogurt to it.
Then add oil followed by milk and vanilla essence. Stir it well with a hand whisk until the sugar is completely dissolved. Mix well so that the oil is mixed evenly.
Once all the wet ingredients are mixed well, add the sieved ragi whole wheat flour mixture to the wet ingredients. Gently fold everything well for 10-15 seconds. Don't over mix. The final mix should be slightly thick. Adjust with milk to get the desired consistency if required.
Grease a cake pan with oil. Pour the mixture to the cake tin and gently tape it. Bake in the preheated oven at 180 deg C for 35 -40 minutes (baking time depends on the size of the cake pan and oven) at 180° Celsius till a toothpick inserted in the center comes out clean. Allow cooling in a tray for 5 minutes once done.
While the cake is baking, prepare the chocolate sauce. In a saucepan, take ¼ cup of cocoa powder, ½ teaspoon of vanilla essence, 1.5 cups of milk and ¼ cup of brown sugar. Mix it well with a whisk.
Finally, add 1 tablespoon of butter and heat the mixture in low medium flame until a sauce consistency is reached. It may take 10 minutes. Switch of the flame once done and cool it down completely.
Once the cake and chocolate sauce is completely cooled down, spread the sauce over the cake gently with a spoon. Then cut the cake into pieces, sprinkle choco chips if preferred.