Preheat oven to 170°C, 325°F or gas mark 3. For the cake, grease the bottom and the side of a 9-inch round cake tin and line with parchment paper.
Beat the egg whites in a medium bowl with an electric mixer on high speed until soft peaks form. In a separate bowl add the flour, sugar, sour cream, melted butter, Matcha Blend and baking powder and mix with an electric mixer on low speed to combine. Then beat for a further two minutes on medium speed. Gently fold in the egg whites until combined. Pour the batter into the prepared pan.
Bake in the oven for 35-40 minutes, or until a knife inserted in the centre comes out clean. Let the cake cool in the tin for 10 minutes. Then remove from the tin and allow to cool completely on a wire rack.
To make the frosting, place all the ingredients in a metal bowl and whisk they until they are all well combined. Fill a saucepan with water to about 500ml. The saucepan should be large enough for the mixer bowl to “rest in it” but without touching the bottom of the saucepan. When the water is simmering place the bowl in the saucepan and whisk the mixture until it reaches 75°C. Remove from the saucepan and allow to cool stirring occasionally.
Slice the cake in half horizontally to form two thin layers. Place one of the layers on a serving plate and spread with lemon curd. Top with the second cake layer. Frost the top and sides of the cake with the frosting. Lightly garnish with grated lemon peel if desired.
Matcha is a green tea powder that can be found in natural food stores, or the organic or tea sections of some supermarkets or online specialty stores.