Soak dates in 3/4 cup of warm milk. After 2 hours deseed them.
Dry ingredients: Sieve whole wheat flour, baking soda or Eno. Add chopped walnuts and mix well. Keep it aside.
Now in a grinder add deseed dates with milk (Use same milk in which dates are soaked), jaggery, oil and grind until smooth.
Now mix ground mixture into prepared dry ingredients. Mix them well and if the mixture looks dry add 2-4 tbsp of milk.

Put batter into a greased pan and tab the cake pan on a countertop to release any large air bubbles.
Put cake pan into the preheated pressure cooker and bake it for 60 minutes.

Tips:

Adjust jaggery according to your taste.
Use room temperature or cold milk. Don’t use hot milk otherwise milk may curdle.
The acidity in the jaggery will curdle the milk if milk is hot.
If Baking in Oven: Bake in the preheated oven at 180 deg Celsius or around 360 Fahrenheit for about 60 minutes. The baking time may vary. So keep checking the cake after 40 to 45 minutes.

Pressure Cooker Method:
Grease the baking tin with oil and dust it by sprinkling whole wheat flour over greased surface evenly. Pour the mixture into the greased baking tin.

Important Notes:

Use a baking tin of 6 inches in diameter, so that it can easily get into the pressure cooker.
Use a 5-liter pressure cooker. It should be big enough to hold the baking pan.
Baking Tin should be made of Aluminium.

If baking tin comes into direct contact with the base of the pressure cooker, it can burn (overbake) the cake. So use pressure cooker container which has big holes in it or any aluminum small stand.
No gasket and whistle are needed. Remove them before baking.
Don’t add water to the pressure cooker.

How to Bake In A Pressure Cooker?

Heat the pressure cooker on high heat for 2 minutes.
Use sea salt or sand as a layer inside a cooker. Put a steel stand on a layer of salt or sand. Now put the cake tin containing the cake batter (Do not put water inside the pressure cooker).
Close the pressure cooker lid and do not put the whistle on the lid. Remove the gasket (rubber ring) from the lid. Lower the flame after 2 minutes.

Let it bake for 60 minutes on low flame. After 60 minutes check cake with a knife, if the mixture does not stick, it means the cake is done. If batter still sticks to knife then again bake for another 10 minutes. Do not open the lid immediately.
Eggless Pressure Cooker Cake is ready.