Preheat oven to 180℃ (350℉). Line two 18 cm/ 7 inch cake pans with circles of baking paper. Grease very well inside the pans.
In a small bowl combine the cocoa powder and the hot water. Mix well to dissolve the cocoa powder.
In a large mixing bowl combine sifted flour, starch, baking soda, baking powder, salt and sugar. Mix with a hand whisk to blend the ingredients.
Add the soft butter and mix with an electric mixer at a low speed until fine crumbs are formed. Gradually add the oil, mixing at a low speed.
Add eggs and whisk for few seconds, until smooth. Add vanilla, cocoa mixture and yogurt. Mix after each addition at a low speed, until combined.
Transfer the batter into the prepared pans dividing it in two relatively equal parts.
Bake for 40-50 minutes or until a toothpick inserted in the centre of each cake comes out clean.
Let the cakes cool for 5-10 minutes and then remove them from the pans: carefully run a knife around the edges, then hold the pan with kitchen gloves and invert it on top of a wire rack or wooden board.
The cake layers should be completely cold before starting to prepare the filling and the frosting and before assembling the cake.
Make the raspberry filling:
Combine all ingredients in a small saucepan over medium heat and stir. When the mixture starts simmering, lower the heat. Stir for 2-3 minutes, until the mixture thickens.
Remove from heat and let cool for 15 minutes before using.
Make the Chocolate Mousse:
Melt the chocolate together with a small part of the cream on a double boiler. Let the mixture cool for a few minutes.
Whip the remaining cream together with the powdered sugar in a large mixing bowl.
Carefully fold the melted chocolate into the whipped cream, adding just small quantities at a time. Make sure the mousse is uniform. If it is too runny to assemble the cake, place it in the fridge for 30-60 minutes but not more.
Assemble the cake:
Using a long serrated knife cut each cake into two equal layers.
Place one layer on a serving plate/ cake stand. Use a piping bag to pipe around the edges some mousse. In the centre spread about ⅓ of the raspberry filling. Continue with the rest of the layers. You need to have about ½ to ⅓ of the mousse for the top and the sides of the cake.
Cover the cake all around with the chocolate mousse.
Refrigerate for at least 4-6 hours before serving.
Decorate with fresh berries.
Notes
Store the cake for up to 4 days in the fridge or freeze the individual slices in airtight containers for up to 2 months.
If you have time, refrigerate the cake layers before assembling the cake, to make cutting them easier.
You can also freeze them for up to 2 months, covered well in plastic wrap, if you don’t need the cake immediately.
The mousse frosting can be prepared 1 day in advance. You need to whip ip before assembling the cake.