Preheat the oven to 350º F/ 180º C. Grease very well a 9 inch/ 24 cm round pie dish or similar pan.
In a large bowl combine flour, baking powder and sugar and whisk very well.
Add the eggs and whisk gently to combine. Add vanilla, yogurt and oil. Continue whisking slowly, until the batter becomes smooth.
Pour half of the batter in another bowl (use a scale or a ladle). Whisk in the cocoa powder and the milk into one part.
Use a large serving spoon or a measuring cup (⅓ or ¼ cup) or just a bigger tablespoon to take from the white batter. Place it right in the middle of the prepared pan. With another spoon or cup take from the chocolate batter and place it right in the middle of the first one. Make sure the second batter doesn't touch the edges of the first one.
Continue the same process until you finish both batters. Your pan should look like it has many circles one inside another.
Whenever necessary, shake gently the pan to help the batter spread easier and make the circles larger.
Bake for 35-40 minutes or until a toothpick inserted in the centre comes out clean.
Let the cake cool until almost room temperature before glazing it.
Take the sour cream out of the fridge at least 15-30 minutes before melting the chocolate. Place it in a bowl and whisk it until smooth.
After melting the chocolate (preferably on a double boiler), mix it with a spatula for 5 minutes to cool down.
Add a spoonful of sour cream to the chocolate and mix very well. Repeat the process 2-3 times and then mix the rest of the cream into the chocolate.
Pour the glaze on top of the cake. Serve immediately or refrigerated.
Store the leftovers for 3-4 days in the fridge, covered with cling film or in airtight containers.