Preheat oven to 350°. Cream butter and sugar until fluffy. Add eggs and lemon zest; mix well. In another bowl, combine flour, baking powder and salt; add to creamed mixture alternately with milk, beating well after each addition.
Spread batter into a greased 13x9-in. or 3-qt. baking dish. Slice pear halves; arrange slices in rows on top of batter. Mix topping ingredients until smooth; spread over pears. Bake until a toothpick inserted in center comes out clean, 30-35 minutes.