Line a 12-section bun tray with paper cake cases. Brush a baking sheet lightly with oil. Put 150 g (5 oz) of the caster sugar in a small, heavy based saucepan with the water and heat gently until it has dissolved. Bring to the boil and boil rapidly until the syrup has turned to a pale golden caramel. Immediately stir in the flaked almonds. When coated, turn out on to the prepared baking sheet, spread in a thin layer and leave until cold and brittle.
Snap half the praline into jagged pieces and reserve. Process the remainder in a food processor until ground.
Put the butter, remaining caster sugar, vanilla extract, eggs, flour and baking powder in a bowl and beat with a hand-held electric whisk for about a minute until light and creamy. Stir in the ground praline. Divide the cake mixture between the paper cases.
Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 20 minutes or until risen and just firm to the touch. Transfer to a wire rack to cool.
Beat together the butter and icing sugar with the hot water in a bowl until pale and creamy. Spread over the cakes using a small palette knife. Decorate with the praline pieces.