Place 16 mini silicone muffin cases on a baking sheet.
Put the butter, sugar, flour, orange rind and egg in a bowl and beat with a hand-held electric whisk until light and creamy. Divide among the cases.
Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 10–12 minutes until risen and just firm. Leave in the cases for 2 minutes then transfer to a wire rack to cool completely.
Use a teaspoon to scoop out (and discard) a little of the sponge from the top of each cake to make a small cavity. Put the marmalade in a small saucepan with the measurement water and heat gently until the marmalade has melted. Leave to cool slightly, then spoon a little onto the top of each cake. Leave to set.
Melt the chocolate (see