Topping: Preheat oven to 180ºC . Melt butter in an 20 cm x 20 cm pan in the oven. Mix in brown sugar and spread evenly over pan bottom. Drain juice from pineapple into a measuring cup and reserve. Spread drained pineapple evenly over the butter mixture. Cake: Cream butter, add sugar and mix thoroughly. Stir in rosemary. In a separate bowl, combine quarter cup of the reserved pineapple juice with sour cream, vanilla extract, and beaten egg. In another bowl, combine dry ingredients. Add dry ingredients to butter and sugar mixture alternately with liquid mixture, mixing well after each addition. Quickly fold in chopped nuts. Spread batter evenly over topping in pan. Bake about 25 minutes until cake tests done. Cool on wire rack 10 to 15 minutes before turning out onto plate. Top with whipped cream to serve, if desired.