In a bowl, combine flour, cornstarch, and sugar. Rub butter into dry ingredients until crumbly and the size of rice grains. Blend in water.
Pat dough into an ungreased 9”x9” square baking pan. Press bottom of another pan the same size against crumbs, making surface as even as possible. Refrigerate 15-30 minutes.
Preheat oven to 325°.
Prick shortbread with a fork in about a dozen places. Put pan on a baking sheet and place in oven. Bake until golden, about 45 minutes. If edges brown too fast, cover loosely with strips of foil.
Place pan on a rack and let cool to lukewarm. Cut shortbread into 16 squares. Remove with a thin spatula and let cool completely. Drizzle with dulce de leche. Store in cookie tins, placing waxed paper between layers.