For the topping, place the 60g butter in a 23cm round cake tin. Place in the oven while it's preheating till the butter is melted, about 4 to 6 minutes. Remove from the oven and sprinkle the brown sugar over the butter, then arrange pear slices on top of the sugar so that they're overlapping slightly. Set aside.
Combine the icing sugar and butter in a mixing bowl and beat well. Add eggs one at a time, then the vanilla, beating well after each addition. Add flour, polenta, baking powder and salt, and stir in with a spatula or wooden spoon till just smooth.
Carefully spoon the cake mixture over the pears in the tin, taking care not to disturb the pears. Smooth the top with a spatula.
Bake in the preheated oven for 50 to 65 minutes, or till a skewer in the centre comes out clean. Remove from oven and cool in the tin for 10 minutes on a wire rack. Carefully invert onto a serving plate and enjoy warm with custard or cream, if desired.