In a mixing bowl, combine the whole wheat flour, cocoa powder, baking powder, baking soda, and salt. Mix well.
In another bowl, whisk together the yogurt, milk, oil, powdered sugar, and vanilla extract until smooth.
Gradually add the wet mixture to the dry ingredients, stirring until just combined. Be careful not to overmix. Fold in chocolate chips or nuts if using.
Grease a heavy-bottomed pot or pan. You can also use a cake tin lined with parchment paper.
Pour the batter into the greased pot or pan and level the top.
Place a stand or a trivet at the bottom of the pot to keep the cake pan elevated. Cover the pot with a lid and heat on low flame for about 30-40 minutes. Check for doneness with a toothpick; it should come out clean.
Once done, remove the cake from the pot and let it cool on a wire rack. Slice and serve, optionally garnishing with icing sugar or melted chocolate.