Measure 2 1/2 cups flour into a mixing bowl and add the yeast, sugar and salt.
Pour in the hot water and the butter, beat 50 strokes with a wooden spoon or 2 minutes with a mixer flat beater.
Fold in the egg whites by hand or with the flat beater.
It will take a few moments for the dough to absorb the egg white.
Add flour, 1/4 cup at a time till the mixture is no longer moist but soft and elastic.
If it continues to be sticky, add sprinkles of flour.
Lift the dough from the bowl and place on a lightly floured work surface, or if in the electric mixer, attach the dough hook.
This is a soft, dough that will be slightly tacky, but not sticky. Knead by hand with a push-turn-fold rhythm, adding sprinkles of flour if the moisture breaks through. Knead for 10 minutes.
If kneading by hand, occasionally lift the dough above the work surface and bring it crashing down to speed the development of the gluten network.
If in a mixer, add sprinkles of flour if the dough sticks to the side of the bowl.
Place the dough in a greased bowl, cover with plastic wrap (or a towel) and put aside to rise at room temperature to double in bulk (about 1 hour, unless using rapid rise yeast).
Divide the dough into equal parts and roll each under the palm of your hands into a 15-inch long baguette.
Lay the dough either in French bread pans or directly onto a prepared baking sheet (greased and lightly sprinkled with corn meal). Keep the loaves apart so they do not touch during rising or baking.
Mix the cornstarch, 1/2 t salt and 1/3 cup water - brush the loaves with this mixture and sprinkle with sesame seeds (or poppy seeds) if desired.
Let rise, uncovered, till doubled in bulk, about 1 hour.
Preheat oven to 350 20 minutes before baking and place a pan on the floor of the oven below the bottom rack. Five minutes before baking carefully pour 1 cup hot water into the pan to create steam for a crispy crust (be careful not to burn yourself with rising steam!).
Bake in the moderate oven for 20-30 minutes, or till the crust is risen and golden brown. (to test, turn one loaf over and thump with the forefinger to make certain it sounds hollow - if not, return to the oven for a few minutes).
Remove from the oven, remove from pans and place on a metal rack to cool.