Line a 9″ by 13″ baking tray with parchment paper. Set aside.
Remove 3 eggs from the fridge. Separate the egg whites and egg yolks, ensuring the egg whites are added to a clean mixing bowl. Leave at room temperature.
Preheat oven to 375°F/191°C.
Fit the mixing bowl with the egg whites to a stand mixer with a whisk attachment. Gradually whisk on low speed until foamy. Add in the cream of tartar. Increase the speed gradually and add in the 30g sugar in small increments.
Beat until the egg whites reach soft peaks (when the whisk it pulled out, there will be a slight curve to its end). Set aside.
Add the earl grey tea to the warmed milk and let it steep for 5 minutes.
Place the egg yolks in a large bowl. Add in 30g sugar and whisk together.
Next, add in the oil, vanilla (or bergamot) extract and cooled earl grey milk and whisk until combined.
Sift in the cake flour over the wet ingredients. Use a spatula to mix together.
Take ⅓ of the whipped egg whites and fold it into the batter. It’s OK to be a little heavy-handed for this first fold since we will add more of the egg whites later.
Next, add another ⅓rd and continue to fold. Repeat with the remainder of the egg whites, until the batter is light and homogeneous.
Transfer the cake batter to a 9″ by 13″ baking tray and smooth out the top with an offset spatula. Tap the tray a few times to remove any trapped air bubbles.
Place into the oven and bake at 375°F/191°C for 13-15 minutes, until the top is dry and springy to the touch. (Don’t overbake or it will be dry).
Remove from the oven and carefully drop it from a height of about 1′.
Transfer the cake over to a cooling rack, with the parchment-side down.
Make the Blackberry Earl Grey Cream:
Blend the blackberries together with the cream until pureed and smooth. (Strain the puree if you prefer a smooth consistency in your cream).
Add the whipping cream, to a large chilled bowl. Whisk until the cream starts to thicken. Add in the icing sugar and pureed blackberries and continue to whisk until the cream stiffens.
Add the reserved earl grey tea leaves (if you wish) from earlier into the cream and fold to incorporate. Note: if the tea leaves are coarse, you may wish you omit. I used a finely ground tea leaf.
Tear off a new large piece of parchment paper.
While the cake is still slightly warm, (not too warm or the cream will melt), but not cool so that the cake won’t roll.
Carefully flip the cake onto the new parchment paper, with bottom side facing up. Peel the old parchment paper off the cake.
Using an offset spatula, spread the blackberry earl grey cream over the surface of the cake.
Carefully roll up the cake and seal the ends of the parchment. Place into the fridge to chill for 1-2 hours.
Slice with a serrated knife and serve.
Note: Cake can be kept in an airtight container for up to 1 week.