There is a recipe in Forever Summer for Coconut Slab. The recipe is for quite a large cake, and I'm afraid i've never tried it so cannot vouch for it, though it is on my list of things to make! I'm not sure where you get good coconut essence from, but have heard Ms Lawson recommend Malibu as a good alternative!
350g soft unsalted butter
350g caster sugar
6 eggs
400g SR flour
50g cornflour
1 tsp coconut essence
100g creamed coconut cut from a slab, put into a measuring jug and filled up to 300mL mark with freshly boiled water.
For icing:
1kg icing sugar
approx 150mLs milk
30g unsalted butter
1 tsp coconut essence
6-8 tbsp desiccated coconut to sprinkle
33cm x 24cm x 6cm tin or roasting pan lined with baking parchment.
Preheat oven to Gas 4/ 180C
Cream butter and sugar. Beat in eggs one at a time, alternating with dollops of the flour mixed with the cornflour. When all is combined, stir in the coconut essence. The creamed coconut should have dissolved into the water - give it a quick whisk then beat into the batter. Pour into the lined tin. Bake for 45 mins until sponge is springy and coming away from sides of pan. Cool in pan for around 20 mins, then turn out onto wire rack.
When cool, make icing. Melt all ingredients (except desiccated coconut) in a saucepan over a low heat. Once you have a cohesive, gleaming mixture, pour over the cake. Should be thick enough to cover the top of the cake without dribbling off the sides - add more milk or icing sugar if it is too thick/ too runny. Sprinkle with desiccated coconut.