Preheat the oven to 160C Fan. Grease and line a 23cm (9-inch) square tin with non-stick baking paper.

In a large mixing bowl, whisk the egg whites with an electric whisk for about 3-4 minutes or until you have soft peaks. Gradually add half (75g) of the sugar a spoonful at a time, whilst continuing to whisk. This should take about 2 minutes. Once all the sugar has been added and the meringue has formed stiff peaks, set aside. 

In a separate large mixing bowl, whisk the yolks with the remaining 75g of the sugar and vanilla extract for about 2-3 minutes until pale and thickened. Add the milk and whisk for about 30 seconds to combine. 

Sift in the flour, baking powder and salt. 

Slowly and gently fold in the meringue a little at a time - you need the mix to keep in as much air as possible. 

Pour into the lined tin and tap gently on the work surface a few times to get rid of any air bubbles. Bake for about 35 minutes - or until the sponge is lightly golden and a skewer comes out clean. Leave to cool for 10 minutes. 

Meanwhile, make the coffee soak: whisk together the coffee, condensed milk, cream and liqueur, if using. Set aside. 

Use a skewer to make lots of holes across the surface of the still-slightly-warm sponge. Pour over half of the coffee soak. Wait for it to be absorbed before pouring over the remaining mixture. Cover and chill in the fridge for at least 4 hours - ideally overnight.

When ready to serve, prepare the topping: whisk the double cream with the icing sugar and vanilla extract until you have soft peaks. Fold in the mascarpone.

Remove the cake from the tin and set onto a flat plate. 

Using a palette knife or the back of a spoon, spread on top of the chilled cake. Dust liberally with cocoa powder.

Serve straight away or chill until ready to serve.