Preheat the oven to 180°C (350°F), Gas Mark 4 and grease and line a 20cm diameter springform tin with baking parchment.
To make the sponge, beat together the eggs and sugar until they are pale and fluffy. Sift the flour and baking powder into the bowl and fold gently into the egg mixture, until you are left with no lumps.
Pour the mixture into the prepared tin and bake on the middle shelf for about 20 minutes or until a skewer inserted into the centre of the cake comes out clean. Leave the cake to cool slightly in the tin and then turn it out onto a wire rack.
When the cake is completely cool, use a bread knife to divide the cake into two even layers. Sandwich the layers with the strawberry jam, reserving a couple of tablespoons for the top and set aside on a cake stand.
To make the frosting, place the egg whites, sugar, vanilla extract and cream of tartar in a large heatproof bowl sitting over a pan of gently simmering water.
Using a hand-held electric whisk, beat on low until the sugar has dissolved completely (check with the back of a spoon
against the bowl). Remove the bowl from the heat and continue to beat for 6 minutes until the mixture has transformed into soft and glossy peaks.
Fill a piping bag fitted with a 3mm nozzle with half the marshmallow frosting and then use a palette knife to spread the remaining mixture all over the cake.
Pipe the other half of the frosting in two rows all around the outer edge of the top of the cake and then carefully spread a little jam into a disc shape on the centre of the cake.
Sprinkle the outer edge of the jam disc with coconut and then gently press the remaining coconut up the side of the cake.