Dough: in a bowl of a mixer with a dough hook put flour, yeast, butter, sugar, eggs, salt, vanilla, yogurt and milk and knead on low speed for 10-12 minutes, until the dough is uniform, soft and slightly sticky.
Cover the bowl and let rise for about an hour or until it doubles in volume.
Cheese and chocolate filling: in medium bowl mix ricotta, sugar and vanilla until blended.
Add egg, egg yolks and cornstarch and continue mixing only until absorbed in the mixture. Refrigerate until assembly of the cakes.
Assemble the cakes: divide the dough into 2 equal pieces and grease the pans.
On a floured surface, roll each part into a rectangle about 1-2 cm thick.
Spread half of the cheese filling in a thin layer. Sprinkle with half the amount of chocolate chips.
Roll tightly into a roulade. Cut into 1-2 cm rolls and place in the pan.
Repeat with the second part of the dough and the remaining filling.
Cover the cakes and let rise for about 30-40 minutes, or until almost double the volume.
Preheat oven to 170c degrees.
Gently brush the cakes with some beaten egg.
Bake for about 35-45 minutes or until the cake golden and set.
Cool for about 1/2 hour at room temperature before serving.
Chocolate sauce: chop the chocolate and put in a small bowl.
Add in cream and melt together in the microwave or over double-boiler, until the mixture is smooth and completely melted.
Serve the cake with chocolate sauce on top.
You can freeze the cake for up to 2 weeks.
Add the chocolate sauce just before serving.
Instead of dark chocolate you can use milk or white chocolate.
The babka is at its best when fresh from the oven, so it's recommended to serve warm.