Put the single cream, vanilla essence and chocolate in a saucepan and heat gently, stirring, until the chocolate melts. Remove from heat.
Whisk the eggs and sugar together, then whisk into the chocolate mixture. Spoon into 6 small cups, espresso cups are ideal. (the quantity and size of your pots depends on the size and shape of your crock pot).
Stand the cups in the crock pot* with enough boiling water to come half way up the sides of the containers. Cover and cook on low for 1 and 1/2 to 2 hours until just set. Don't overcook.
Remove from the crock pot, leave to cool, then chill.
Whip the double cream until peaking and spoon on the top of each chilled pot of chocolate. Dust with sifted cocoa and serve.
*I use crumpled up foil as a nest in the bottom of the slo-cooker.