Preheat oven to 180 degrees Line the bottom of a 22-24 cm cake pan with a circle of baking paper and grease it well.
In a bowl combine flour, salt, baking soda, baking powder and cocoa and stir to blend. Set aside.
In a large mixing bowl beat together olive oil and sugar. Add yogurt and vanilla and continue mixing until homogeneous. Gradually add the dry ingredients, mixing carefully until just absorbed.
Pour the batter into the prepared pan and bake the cake for about 40-50 minutes or until a toothpick, inserted in the centre comes out clean. Remove the pan from the oven and let cool for 5-10 minutes, before removing the cake.
Allow the cake to cool completely before topping it with the hot chocolate.
Hot Chocolate sauce:
In a small bowl combine 50 ml milk, the cornstarch and cocoa powder and mix until dissolved.
In a saucepan over medium heat combine 120 ml milk and the sugar and bring to boil. When it starts simmering, lower the heat and add the cornstarch mix, while stirring continuously to avoid burning. When the mixture starts thickening, remove from heat.
Using a toothpick make small holes on top of the cake. Pour evenly the hot chocolate sauce. Serve immediately.
Any hot chocolate leftovers can be consumed directly as a drink or can be refrigerated, then reheated and used as an additional cake topping. The cake can be stored in a aright container in the fridge for up to 4 days.