Break the dark chocolate into small pieces and melt in a heatproof bowl set over a pan of simmering water, ensuring the bowl doesn't touch the water. Stir until smooth, then remove from heat and let cool for 5 minutes.

Dissolve the instant espresso powder in the hot water and stir into the melted chocolate until well combined.

Add the egg yolks to the chocolate mixture one at a time, stirring well after each addition until smooth and glossy.

In a clean, dry bowl, whisk the egg whites with a pinch of salt until soft peaks form. Gradually add the caster sugar and continue whisking until stiff, glossy peaks form.

Gently fold one-third of the whipped egg whites into the chocolate mixture to lighten it, then carefully fold in the remaining whites until no streaks remain.

In a separate bowl, whip 150ml of the double cream with vanilla extract until soft peaks form. Reserve the remaining 50ml cream for topping.

Fold the whipped cream into the chocolate mixture until just combined, being careful not to deflate the mousse.

Divide the mousse evenly among four 200ml glass jars or serving glasses. Cover and refrigerate for at least 2 hours or until set.

Just before serving, whip the remaining 50ml cream until soft peaks form and dollop on top of each mousse jar.

Garnish with chocolate shavings and fresh raspberries. Serve chilled.