Mix chocolate and milk in a pan and heat till chocolate is melted and dissolved in milk. Whisk together all purpose flour, baking powder and salt in a bowl.
Beat butter and sugar in a bowl with the electric blender till light and fluffy. ( Approx 3-4 minutes on high speed )
Add egg, one at a time and blend well.
Add ⅓ rd of the flour to the bowl and mix.
Add orange juice and mix.
Add half of the remaining flour and mix.
Add curd and mix. Add the remaining flour and mix.
Pour half of the batter in the bowl with chocolate and mix well.
In the remaining batter, add orange zest and vanilla extract. Pour half of the chocolate batter in the prepared pan.
Pour the orange vanilla batter on top of it.
Finally pour the remaining chocolate batter on top. Swirl a little with a knife. ( do not over swirl ) Bake the cake for 25-30 minutes or till a knife comes out clean.
Remove the cake from the oven. Let it cool in the pan for 5 minutes.