Preheat oven to 160C and grease a 20cm non-stick cake tin with a removable base
Melt the butter and chocolate in a bowl over a pot of simmering water, stirring occasionally until melted. Remove from heat but keep over the pot of warm water.
Meanwhile, whisk the egg yolks in a large bowl with half the caster sugar until pale and creamy.
In a separate clean bowl, whisk eggs whites until stiff, ensuring to add the remaining caster sugar in three additions. The mixture should resemble a glossy meringue
Fold melted chocolate mixture into the egg yolk and sugar mixture and then carefully fold in the meringue in two goes.
Pout mixture into the prepared tin, tap on a work surface to remove bubbles and then bake for an hour or so (could be done around the 45min mark) until a skewer inserted comes out clean or with very little crumbs
Let cake cool in the tin
To make the glaze, melt chocolate in a bowl in the microwave, and then remove and whisk in the hot coffee until you have a smooth glaze.
Remove cake from tin (i left mine on its base) and pour the glaze into the crater of the cake and top with decorations if desired.
Cool completely in the fridge before eating