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.