Prepare the cakes: Preheat the oven to 175°C fan. Line the bottoms of two 20-cm round pans with parchment paper.
In a large bowl, cream the butter until softened, then add the oil. Beat until well combined.
Sift the flour, cocoa, baking powder, baking soda, and salt into the bowl. Add the sugar and milk, and continue beating until combined.
Add the eggs one at a time, beating well after each addition.
Finally, pour in the boiling water and beat well.
Transfer the mixture to the cake pans and bake in a preheated oven (175°C fan) for 30–35 minutes or until a toothpick inserted into the center of the cake comes out clean.
Run a small knife around the edges of the cakes and invert them onto a wire rack. Remove the parchment paper and cool.
Prepare the syrup by placing the sugar and water in a small saucepan. As soon as it comes to a boil, I remove it from the heat and add the cognac. I set it aside to cool.
To prepare the red fruit filling, dissolve the cornstarch in water in a small bowl. Place the fruit in a pan along with the sugar and the dissolved cornstarch. Bring to a boil and simmer for 3–4 minutes until the mixture thickens but isn’t completely dry. Remove from the heat and divide the mixture into three equal parts.
For the filling and frosting, beat the mascarpone cheese with the heavy cream, powdered sugar, and vanilla in a mixer until you have a fluffy whipped cream.
To assemble the cake, cut the two cake layers in half to create 4 discs. Place the first disc on a serving platter and brush it with ¼ of the syrup. Continue with a layer of whipped cream. Pipe a border of whipped cream all around the edge and fill the cake with 1/3 of the red fruit filling.
Cover with the second cake layer and repeat the process two more times. Finish with the fourth cake layer and brush with the remaining syrup.
Cover the entire cake with whipped cream and decorate with a piping bag and fresh red berries.
Let the cake chill in the refrigerator for at least 4 hours before serving.