Preheat oven to 350 degrees. Grease a 9-inch square pan. Cut the dates into small pieces. Melt the butter and stir in the sugar until dissolved. Remove from heat. In a large bowl, beat the eggs until light, add the vanilla, lemon zest and juice. Beat in the butter mixture, then the flour, salt and baking powder. Scrape the bowl, then mix in the dates and chocolate chips. Transfer the batter to the prepared pan and bake for 30 to 35 minutes, or until golden brown. Remove from the oven and cool on a rack.