Pre-heat the oven to 160°C/140°C fan assisted oven/gas mark 3/320°F and line and grease 2 x round 8 inch cake tins.
Beat together the sugar and butter until pale, light and fluffy.
Add the eggs one at a time, beating on a low speed.
In a measuring jug whisk together the whole milk and vanilla extract and set aside for a moment.
Whisk together the flours, baking powder and sea salt in a large mixing bowl until completely combined.
Add the flour mix alternately with the milk, adding the flour in three additions and the milk in two (begin and end with the flour), scrape down the sides of the bowls as needed and mix until just combined.
Divide between the cake tins and bake for 45 minutes or until an inserted cocktail stick comes out clean.
Swiss Meringue Buttercream
Heat egg whites and caster sugar in a bain marie, stirring constantly until the sugar has dissolved and the temperature has reached 71°C.
Remove the egg whites and sugar from the heat and pour into a stand mixer with whisk attachment. Whisk until the mixture forms stiff peaks.
Change the attachment to a paddle attachment. On a low speed add the butter slowly cube by cube. When you have almost added all the butter the mixture will look curdled. Do not fret – this is supposed to happen and just means you are nearly done. Just continue to add all the butter. Once the butter is totally incorporated the frosting will miraculously become a smooth velvety consistency.
Add the salt and vanilla extract. Mix until thoroughly combined.