Take ¼ cup milk in a microwave safe bowl and micro for about 20-30 seconds. You can also heat milk on a medium flame. Soak dates in it and set aside for about 30 minutes or till dates become soft. If you are using black dates, they will soften quickly.
Take soaked dates in a mixer jar along with 1/3 cup brown sugar and grind to a fine paste.
Now take this dates paste in a wide bowl. To it, add remaining ½ cup milk and oil and whisk well.
Now sieve and add all the dry ingredients - maida, wheat flour, baking powder, baking soda and pinch of salt. Mix well till everything gets combined well.
Gradually add about ¼ cup water to get a batter of pouring consistency. Also add chopped walnuts and mix.
Grease the cake tin with butter and line it with a parchment paper. Pour the batter into the tin. Tap 8-10 times to level.
Preheat the oven for 10 minutes before baking at 180 degrees.
Bake for 35-40 minutes at 180 degrees. Be alert after 30 minutes. Baking time will slightly vary for each convection. Insert a toothpick or skewer in the centre. If it comes out clean, cake is ready. If not, then again bake for a few minutes.
Cover with a cloth and allow the cake to slightly cool. Once slightly cool, invert on a plate and remove parchment paper. Be careful while unmoulding the cake.