Preheat oven to 180 C for at least 20 minutes.
Take a mixing bowl and add 1 cup whole wheat flour to the sieve.
Add 1 tsp baking powder to the flour.
To this add ¼ tsp salt.
Add a pinch or ⅛ tsp baking soda to the flour.
Sift everything, mix well and keep it aside.
Take 3 tbsp of yogurt in a small bowl and add a pinch of baking soda. Beat it well with a spoon and keep it aside. After a couple of minutes, the mixture turns frothy because of the reaction between baking soda and yogurt.
Meanwhile, extract juice from passionfruit. For making juice, just take the passionfruit pulp on to a strainer and mash it with the spoon. I got 3 tbsp of juice from 2 passionfruits.
Now take a mixing bowl and add ¼ cup coconut oil and ½ cup sugar. Whisk it well for a minute or so with a hand whisker.
Now add buttermilk-soda mixture to this and mix it once. To this, add 3 tbsp passionfruit juice and whisk it for a couple of minutes.
Now add sifted flour mixture to this wet mixture in three intervals. Every time when you add flour mixture, add 1 tbsp of milk and mix it with light hands.
The final mixture should be of pouring ribbon consistency. If required you can add one more tbsp of milk or passionfruit juice.
Line a baking pan with butter paper and pour the cake mixture into it. You can add passionfruit seeds or pulp as topping as well.
Bake the cake for 30 minutes at 180 C. Check at 25 minutes as baking time depends varies from oven to oven. If a toothpick comes out clean when inserted into the cake, then the cake is ready.
Take cake out of the oven and cool it on a wire rack. Slice the cake when it is cooled completely and the cake is ready to serve!