Preheat oven to 180`c. Next, butter and line a deep 9-inch-diameter cake tin. Stir sugar and water in heavy small saucepan over low heat until sugar dissolves. Increase heat; boil without stirring until syrup turns deep amber, occasionally swirling pan and brushing down sides of pan with wet pastry brush, about 10 minutes. Pour the caramel into the bottom of the prepared tin. Overlap the pears on top of the caramel. Next, prepare the cake. Mix flour, baking powder and salt in medium bowl. Using an electric mixer, beat the sugar and butter in large bowl until creamy. Add eggs and lemon zest and beat until light and fluffy. Combine milk and brandy in small bowl. Mix dry ingredients into batter alternately with milk mixture in 3 additions before finally adding the walnuts. Pour batter on top pears in the pan and even out. Bake the cake in the oven for about an hour, or until golden on top and a skewer inserted into center comes out clean. Immediately prepare the custard according to the directions on the packet. Serve the pear and walnut cake warm drizzled with the hot vanilla custard.