Pour the hot butter over the chocolate and stir quickly until it's all melted together.
Leave to cool for approximately ten minutes before adding the egg yolks, caster sugar and plain flour, then stir well until everything is mixed - at this stage it'll be a little grainy but very dark.
In another large bowl whisk the egg whites until they form stiff peaks. Pour over around half of the chocolate mixture and fold it into the egg whites, making sure you beat lots of air into the mixture. Repeat this two or three times with the rest of the mixture - at this point the mix will be glossy and light.
Pour into one lined 20 cm tin and bake for around 40 minutes. Bear in mind the cake is quite moist and fudgy so may appear a little wet at first. Leave to cool in its tin on a wire rack.
Make your ganache topping by melting the chocolate and the cream in a saucepan over a low heat. Spatula this mix over the cake and serve with raspberries and vanilla ice cream.