Heat the oven to 180°C (160° fan) 350°F gas 4. Line the base and sides of a 30 x 22 cm|12" x 9" roasting tin with non-stick baking paper.
Melt the chocolate in a heatproof bowl over a pan of simmering (not boiling) water. Remove from the heat.
Beat the butter and sugar in a mixing bowl until light and fluffy. Whisk in the whole eggs and yolk until blended. Stir in the chocolate and coffee.
Sift in the flour and baking powder and gently stir into the mixture with the salt, until combined.
Spoon into the tin and bake for 25-30 minutes until the top is firm. Leave to cool in the tin.
For the icing: put the strawberries into a pan with the water and sugar. Bring to a boil and simmer for about 3 minutes until the strawberries are soft. Remove from the heat and mash until pulpy.
Stir the marshmallows into the hot pulp until they dissolve. Stir and leave to cool.
Whisk the cream until it holds its shape, but is not stiff. Fold the cream into the cooled strawberry mixture, then spoon into a bowl and chill for1- 2 hours until thick but still spreadable.
Cut out heart shapes from the cake, using a heart-shaped cookie cutter.
Place a spoonful of the strawberry mousse on top of the hearts and spread fairly thickly over the cake. Chill until the mousse has set.
Decorate with sugar hearts and chocolate flakes.