Base-line the mini sandwich tin with little discs of baking paper.

Pre-heat the oven to 170 C Fan/180 C/350 F.

Fill a bowl of water with warm (NOT hot) water and place the eggs (in their shells) in the water to warm a little while preparing the other ingredients.

Weigh and mix together the flour, salt and baking powder and set aside.

Measure the xanthan gum into a small pot and set aside.

In a large bowl, use an electric whisk to cream the softened butter with the caster sugar and vanilla extract until pale and fluffy. Beat for a good 2 to 3 minutes.

Separate the eggs, placing the yolks in the bowl with the creamed sugar-butter mixture and the whites separately in a small cup/bowl.

Beat the yolks into the mix at low to medium speed until the mixture is smooth and even.

Use a fork to lightly beat the egg whites in the cup/bowl until they are loosened and gently airy.

Very gradually add the egg white to the batter mixture a little at a time, gently beating at LOW SPEED between each addition until even.

Scrape down the sides of the bowl intermittently and for the last couple of additions of egg, also add a spoonful of the weighed flour mixture to the bowl.

Once all of the egg is added, transfer the xanthan gum to the remaining dry flour blend and mix together.

Sift about half of the flour into the mixing bowl and gently fold in using a large spoon or spatula.

Next, fold in half of the milk and the remaining flour. Then the remaining milk and fold until the batter is smooth, even and has a soft dropping consistency.

Be careful not to over-mix.

Divide the batter in the cake tins to about two-thirds full in each hole and level the tops with the back of a teaspoon.

Bake for around 20 to 25 minutes until golden brown, a skewer inserted comes out clean and the tops spring back to the touch.

Remove from the oven and leave the cakes to cool in the tins for about 10 minutes, before turning out onto wire racks to cool completely.

While the sponges are cooling, make the buttercream - Beat the butter in a bowl until pale and light.

Add the icing sugar a little at a time and beat through until smooth.

Add the custard powder and vanilla extract and beat through again.

Lastly add a drop of milk, as necessary to loosen the buttercream to piping or spreading consistency.

When the sponges are cold, carefully cut each of them in half and set in pairs ready to sandwich together.

Either spread or pipe the buttercream on the base sponge of each pair.

Spread a layer of jam on the cut side of each top layer and sandwich together with its corresponding buttercreamed base.

Dust the top of the cakes with icing sugar and serve.