Preheat oven to 400ºF (200ºC).
In bowl of heavy-duty mixer and using paddle attachment, combine yeast, sugar, salt and 1 cup (250 mL) of the flour. Add hot water; beat at medium speed for 2 minutes or until smooth. Reduce speed to medium-low. Add eggs, 1 at a time, beating well after each addition. Gradually beat in 2 cups (500 mL) of the remaining flour.
Add butter, a few pieces at a time, beating until completely incorporated. Gradually add remaining flour to make very sticky soft dough. Cover with plastic wrap; let rise in warm draft-free spot until doubled in bulk, about 3 hours.
Gently deflate dough; cover and refrigerate for at least 6 hours or for up to 18 hours. (Dough can be frozen for up to 2 weeks; thaw in refrigerator for 24 hours before proceeding.)
Turn out dough onto lightly floured surface. Using no more than 1/2 cup (125 mL) flour to keep hands floured during shaping, gently knead dough into ball. Divide into quarters. Roll each quarter into 12-inch (30 cm) long rope. Divide each rope into six 2-inch (5 cm) pieces; cut 1 of the pieces from rope into 5 equal lengths. Shape all pieces into balls, making 20 large and 20 small. Place large balls in 20 well-greased brioche moulds or large muffin cups. Using scissors, cut X in top of each. Snuggle small ball into each X. Stir egg yolks with water; brush over brioche. Cover lightly and let rise in warm draft-free spot until doubled in bulk, about 45 minutes.
Bake in centre of oven for about 20 minutes or until golden. Let cool slightly in moulds on racks. Remove moulds; let cool completely on racks. (Brioche can be wrapped individually and frozen in airtight container for up to 12 weeks.)