In a clean bowl, mix well vegetable oil and cocoa powder. Add in milk and mix well. Add in cake flour and mix well. Set aside.

Separate the eggs. Add the egg yolks into the batter and mix well.

Line the baking tray (28*28*2.6cm) with parchment paper or baking mat. Preheat the oven to 302℉(150℃).

Add lemon juice, sugar and vanilla extract into the egg white bowl. Use a hand mixer to beat until soft peaks form. Take a scoop of whipped egg white and gently fold it into the batter until just combined. Pour all the batter into the whipped egg white and gently fold it in until just combined.

Pour all the batter into the baking tray. Smooth it out, give it a tap or two, and send into the oven to bake for 32 minutes.

In a clean bowl, put together heavy cream, vanilla extract, sugar and salt, use a hand mixer to beat until thickened. Keep it cool in the fridge.

Take the cake out of the oven and let the cake cool down on the cooling rack before take it out of the cake pan. When the cake is cool to touch, release the sides of the cake with an icing spatula, turn the whole cake pan upside down on the parchment paper. Take off the pan, and peel off the baking mat.

Slice the cake in half into two equal size rectangle. Spread the cream onto one side of the cake and top with another slice. Let it cool in the fridge for 30 minutes. Slice the cake into several finger sandwiches.