In a saucepan, melt together the chopped chocolate , milk , butter and sugar until the chocolate and butter is melted and the sugar is dissolved. Stir often. Let it come to room temperature.
Preheat your oven to 160 C / 325 F. Butter and flour two 6 inch pans or one 7 inch round pan. Line the bottom with parchment .
Into the cooled chocolate and milk mixture, whisk in the yoghurt and vanilla extract until smooth.
Whisk or sift together the flour, baking powder, baking soda and salt. Add to the batter and whisk until smooth.
Pour into prepared pan or pans and bake at 160 C until a toothpick inserted in centre comes out clean .This will take 48 to 55 minutes in the 6 inch pans and about 70 to 80 minutes in one 7 inch pan. The edges of the cake will come away from the pan.
Cool in pan for 10 minutes , gently remove and cool completely on a wire rack.