In a bowl, using an electric mixer, beat the 1 stick plus 2 tablespoons of butter with the sugar and egg yolk until smooth. Add the flour, baking powder and salt and beat at low speed until evenly moistened, about 2 minutes. Using your hands, knead once or twice to bring the dough together.
Preheat the oven to 350°. Butter a 9-inch square metal baking pan. Pat three-fourths of the dough into the baking pan in an even layer. In a bowl, beat the pecans into the remaining dough. Crumble the pecan streusel on a plate. Refrigerate the crust and streusel for 10 minutes.
In a medium saucepan, combine the chocolate and condensed milk and stir over low heat until melted and very thick, about 3 minutes. Scrape into a bowl. Press a sheet of plastic wrap directly onto the surface and let cool slightly.
Spread the fudgy filling evenly on the dough in the pan and sprinkle with the coarse salt. Scatter the pecan streusel on top. Bake in the lower third of the oven for 40 to 45 minutes, or until the top is golden. Let cool completely.
Run the tip of a knife around the edge of the pan. Cut the square into 18 bars and serve.