Make the Sponge Cake

1. Preheat the oven to 150°C (300°F). Line a 9 x 13 inch (33 x 23 cm) baking pan with a reusable non-stick baking sheet.
2. Heat the oil in the microwave for 45–60 seconds until hot but not smoking. Mix it with the cocoa powder in a large bowl to bloom the cocoa, then set aside.
3. Add the milk to the cocoa-oil mixture and whisk until fully emulsified. Stir in the vanilla extract.
4. Sift in the cake flour and salt. Whisk gently until no dry streaks remain.
5. Add the egg yolks and whisk until the batter is cohesive, silky, and glossy.

Whisk the Egg Whites

1. In a clean, dry bowl, combine the egg whites and lemon juice. Beat on medium speed until foamy.
2. Gradually add the sugar in three stages, waiting 30 seconds between additions for the sugar to dissolve.
3. Increase to medium-high speed and whisk until distinct trails are visible on the surface.
4. Reduce to low speed and beat for 10–20 seconds to “polish” the meringue, breaking down large bubbles into a uniform, stable foam.
5. Stop when medium peaks form — the tips should gently curl over like a bird’s beak.

Fold Meringue into Batter

1. Gently fold 1/3 of the meringue into the yolk mixture to lighten it.
2. Pour this back into the remaining egg whites and fold until fully combined using a whisk and no white streaks remain.

Bake and Cool the Cake

1. Pour the batter into the prepared pan and let it settle, especially in the corners. Smooth the surface with a cake scraper.
2. Bake for 15 minutes, or until the sponge springs back lightly when touched.
3. Remove from the pan, let it sit on a wire rack and  peel off the baking sheet.
4. Once slightly cool, place parchment on top and flip the cake. If you prefer the bottom as the outside, flip it again — it’s up to you.
5. Roll the cake gently with the parchment paper to shape it. Wrap in a kitchen towel to keep it moist and soft. Let it cool completely on the countertop.

Make the Filling

1. Sift the cocoa powder into the icing sugar. Add the salt and mix until well combined. Set aside.
2. Heat the double cream, vanilla extract, and coffee powder in a saucepan over medium-low heat, stirring constantly until the coffee dissolves. Let it cool slightly, then cover and chill until completely cold.
3. In a stand mixer bowl, combine the chilled mascarpone cheese and cocoa-sugar mixture. Roughly mix with a spatula to prevent sugar from flying.
4. Attach the whisk and start mixing, then slowly pour in the chilled coffee-infused cream while the mixer is running.
5. Whisk on medium-high speed until stiff peaks form. Cover and chill until ready to use.

Fill and Roll the Cake

1. Once the cake is completely cool, spread a generous amount of mascarpone cream in the center. Spread the remaining cream evenly over the top, leaving a 1-inch border at one edge.
2. Roll the cake tightly from the opposite end, using the parchment paper to assist.
3. Wrap the roll in parchment paper and refrigerate for 30 minutes to set.
4. Trim the edges for a clean finish. Place on a serving plate and slice.

Decoration (Optional)

1. Pipe any remaining cream using your preferred tip.
2. Lightly dust with cocoa powder for a polished look.