Adjust oven rack to middle position; heat oven to 350° F. Line a 9 by 9-inch baking pan with parchment paper and set aside.
Whisk flour, baking powder, cinnamon, and salt together in medium bowl; set aside.
Whisk melted butter and sugar together in medium bowl until combined. Add eggs and vanilla and mix well.
Using rubber spatula, fold dry ingredients into egg mixture until just combined; do not over mix. Fold in the chopped apple.
Spread 1/2 of the batter into prepared pan, smoothing top with rubber spatula.
Pour the condensed milk over the batter. Top with the remaining batter. This part can get a little tricky, some of the condensed milk may be peeking through. Don't be afraid to use your fingers to help spread the batter around.
Bake until top is shiny, cracked, and light golden brown, about 45 minutes; do not overbake. Cool on wire rack to room temperature. Remove bars from pan by lifting foil overhang and transfer to cutting board.
While the bars are cooling prepare the glaze. In a small bowl combine the glaze ingredients. Use a fork or whisk to combine. Drizzle over the cooled bars. Cut into 12 large or 16 small(ish) bars and serve.