Heat the heavy cream until steaming, pour it over the chocolate chips and stir until smooth. Freeze the ganache for 20 minutes, then scoop it into 4 balls and keep them chilled.
Preheat the oven to 425 F. Grease four 6 oz ramekins and dust them lightly with cocoa powder.
In a bowl, whisk the flour, cocoa powder, granulated sugar, baking powder, baking soda and salt.
In another bowl, whisk the milk, yogurt, melted butter and vanilla, then stir into the dry ingredients just until smooth.
Fill each ramekin halfway with batter, place a chilled ganache ball in the center, then cover with the remaining batter.
Bake for 13 to 14 minutes, until the edges are set but the centers still look slightly soft.
Let the cakes rest for 2 minutes, run a thin spatula around the edges and invert each onto a plate.
Dust with powdered sugar and serve immediately while the centers are molten.
