Preheat the oven to 180 degrees Celsius/gas 4. Grease and line the base of a 23cm/9in spring-form tin with baking parchment.

Blend the chocolate and sugar in a food processor until a fine powder forms. The omission of flour gives a more intense chocolate flavour. Add the pinch of salt.

Transfer to a large bowl, then add the boiling water, butter, egg yolks and vanilla extract and mix until well combined.

In a clean bowl, whisk the egg whites until stiff peaks form when the whisk is removed. Using a metal spoon, gently fold into the chocolate mixture. Pour the mixture into the prepared cake tin and bake for 45-55 minutes, or until a skewer inserted into the middle comes out clean.

Remove the cake from the oven and allow it to cool in the tin for two to three hours. The cake will collapse and settle as it cools.

Meanwhile, make the Swiss meringue. Place the egg whites and sugar in a large metal or glass bowl.

Set the bowl over a pot of simmering water, and whisk, continuously, until the sugar has completely dissolved and the mixture looks opaque. Remove the bowl from the heat, add vanilla extract, and whip on high speed until completely cool.

Whip in the butter, a tablespoon at a time, until thick and fluffy. If it begins to look curdled, continue to whip until it comes back together, before adding the remaining butter. Fold in the melted chocolate and cocoa and water paste.

If the egg whites and/or the butter are slightly too warm, the butter cream may not thicken properly. This can be easily remedied by placing it in the fridge for 20-30 minutes, then whipping again.

To serve, remove the cake from the tin and place on a serving plate or cake stand. Pile the chocolate Swiss meringue over the top of the cake and smooth it using a palette knife. Scatter with some little Easter eggs, or bunnies, or mini meringues.