Preheat the oven to 350°F. Lightly grease a 9" x 13" pan; line the pan with parchment paper or foil, with the edges of the paper going up the sides. Clip the foil or paper to the pan with metal spring clips if necessary., To make the crust: Whisk together the flour, salt, and baking powder in a small bowl. In a larger bowl, beat together the butter and sugar. Mix in half the dry ingredients, then add the egg yolks. Scrape the sides and bottom of the bowl, then add the remaining dry ingredients, mixing just until the dough comes together., Pat the mixture into the prepared pan. Bake for 15 minutes, until light golden brown. Remove from the oven., To make the filling: Combine the condensed milk, lemon zest, and lemon juice, stirring until blended. The mixture will thicken slightly. Spread it on the cooled crust and set aside., To make the topping: In a clean bowl with clean beaters or whisk, beat the egg whites with the lemon juice until foamy. With the mixer running, add the sugar gradually and beat until thick, and the whites hold a medium peak., Spread or pipe the meringue over the filling, and return the bars to the oven to bake for another 15 minutes, until the meringue is golden brown. Remove from the oven and cool on a rack for 20 minutes., After 20 minutes, loosen the ends of the bars from the pan, and use the parchment or foil to lift the whole recipe straight up out of the pan onto a cutting surface. Now the bars will be much easier to cut into 35 small squares.