Preheat the oven to 350 degrees F.

Grease a 9"x13" pan with softened butter and line with parchment paper. Grease a separate large piece of parchment paper with softened butter and set aside. You'll need this to roll up the cake.

In a small heat-proof bowl, combine the chocolate, oil, vanilla, espresso powder, and salt. Place the bowl over a small pot of simmering water, and stir frequently until the chocolate has full melted. Set aside to cool.

In the bowl of a stand mixer fitted with the whisk attachment, beat the egg yolks and sugar on high speed until doubled in size and the mixture drizzles off the whisk like a ribbon.

Pour the cooled chocolate mixture into the egg yolk mixture and mix to combine.

In a separate bowl, add the egg whites and use an electric hand mixer to beat until stiff peaks form.

Use a rubber spatula to fold ⅓ of the egg whites into the yolk, chocolate mixture to loosen it up, then fold in the rest until fully combined.

Sift the flour and cocoa powder on top of the batter, then gently fold it in until fully combined. Be careful not to deflate those egg whites!

Pour the batter into the prepared pan. Bake the chocolate sponge for about 13-15 minutes or until a toothpick inserted comes out clean. Do not over-bake!

Immediately turn the cake out onto the parchment paper greased with butter. Carefully roll the cake up into a loose swirl. Place in the fridge to chill for 20-30 minutes.

Meanwhile, make the cream filling and ganache.

In a medium bowl, add the mascarpone and sugar. Use an electric mixer to beat on low-medium speed until smooth. Then, add the heavy cream and continue beating until stiff peaks form.

Chop the chocolate and add to a medium bowl. Set aside.

In a small saucepan, heat the heavy cream over low heat and bring to a simmer or about 190 degrees F.

Pour the hot heavy cream over the chocolate, place a plate on top of the bowl, and allow to sit for a couple minutes.

Remove the plate and gently stir with a rubber spatula until the ganache is smooth. Alternatively, you can use an immersion blender.

Assemble the cake immediately.

Over a wire rack, very slowly unroll the cake half way. Spread the mascarpone cream on the inside of the cake, coaxing it up to the center part of the swirl. Roll the cake back up and remove the parchment paper.

Pour the ganache over the cake, then transfer to a serving plate.

Slice the cake and enjoy!