Collect together your equipment (see Recipe Notes below) and ingredients.

Preheat oven to fan 160°C/180°C/350°F/Gas 4 and line a 20cm/8inch square tin with greaseproof paper.

Make the coffee cake slice: dissolve the coffee powder in the boiling water and set aside to cool.

It is important that the butter is at room temperature and soft enough to beat. If it is a little hard, beat it with a whisk before adding the other ingredients. If it is already quite soft, you do not need to do this. For information on softening butter, see here.

Measure all the remaining ingredients, apart from the coffee, into the mixing bowl with the butter. I place my mixing bowl on top my scales and weigh everything in directly. Add the coffee at the end, when it has cooled a little.

Use an electric whisk to beat until combined. This should take less than a minute. Half way, use a spatula to scrape the sides of the bowl and ensure even mixing.

Tip into your lined baking tin and level the surface, ensuring the cake goes into all the corners.

Bake in the centre of a preheated oven for 45 – 50 minutes until cooked. It is cooked when:

- the centre feels springy when lightly touched with your finger and no imprint remains.

- a toothpick inserted into the centre of the cake comes out clean.

- the cake is beginning to come away from the sides of the tin.

Leave to cool in the tin for 5 minutes and then, using the greaseproof paper over-hang, transfer to a cooling rack and set aside until completely cool.

Make the buttercream frosting whilst the cake cools: mix the coffee with the boiling water and set aside to cool.

It is important that the butter is at room temperature and soft enough to beat. For information on softening butter, see here. Add the salt to the butter and whisk until light and fluffy.

Put the mixing bowl on the scales, balance the sieve on the bowl and zero the scales. Weigh in half the icing sugar and sieve. Quickly fold in with a spoon. (This is to stop the icing sugar clouding up and making a mess of the kitchen!) Whisk until light and fluffy.

Repeat with the remaining icing sugar and whisk again.

Thoroughly mix ¾ of the cold coffee into the frosting. Taste to see if you want to add the remaining coffee or any more salt.

Cover the top of the cake with the buttercream and make a swirly pattern with a knife or the back of a spoon.

If using, make milk chocolate curls using a potato peeler – to get these fabulous curls, the chocolate should be at room temperature, not too cold. If it is cold, and you will get much smaller curls but they will still be pretty.  

Serve as is or decorate with chocolate curls or sprinkles and slice into 16 squares.