In a large bowl, sprinkle the yeast over the water.
Let stand for about 5 minutes to dissolve the yeast. Stir in the salt, sugar, oil and 4 eggs until well blended. Gradually mix in the flour. When the dough becomes too stiff to stir, turn it out onto a floured surface and knead for 8 to 10 minutes.
Place dough underneath the bowl to rise until double. Or, you can place the dough in the bowl, and cover with a towel.
Punch down the dough, and divide into 6 or 8 even pieces depending on what shape you want. Remember to take a small piece off and make a blessing (Jewish law).
Roll the pieces into ropes. Braid into two loaves, or one really big 6 piece braid - but only if your oven is large enough. Or, you can make the spiral shape challahs out of each rope. Tuck the ends under, and place on a baking sheet to rise until your finger leaves a small dent when you gently poke the bread.
Preheat the oven to 400 degrees F (200 degrees C).
Whisk together the remaining egg, water and vanilla sugar.
Brush over the tops of the loaves.
Sprinkle sesame seeds over the top.
Bake for 35 to 40 minutes in the preheated oven, until the bread is deep golden brown. Wrap the small piece of dough that was blessed in aluminum foil, and burn in the oven as an offering while the other loaves are baking.
