Heated the oven to 160-170C degrees and prepare your cake cutter with baking paper. Mix flour with baking powder and vanilla and sift them (if using liquid essence add it together with yogurt). Cut the chocolate into small pieces and leave aside. In a deep bowl whisk the butter and olive oil with the sugar, then add eggs one by one. Without stopping the mixer add the yogurt and water and continue beating. Little by little, add flour mixture by continuing to mix to. Once you've added everything and received a fluffy and smooth mixture sent down 1/3 in cakes form. Top and chocolate spread on it, right in the middle, pour the rest of the mixture - so it pushes the chocolate and obtained * smile *. Put in preheated oven and bake 50-60 minutes or until a toothpick comes out dry. Subtract from the oven and leave to cool, then Eat with a smile.