Ornately decorated paska bread is customarily prepared for Easter in Ukraine. The dough is marked with a four-sided cross design and often used as the central focus of Ukrainian Easter baskets.
In a large bowl, stir together yeast, 1 teaspoon all-purpose flour, 1 teaspoon sugar, and warm water; let stand until foamy, about 5 minutes. Add 1 1/3 cups all-purpose flour, high-gluten flour, and milk; stir to combine. Cover with plastic wrap and let stand in a warm place until bubbles form, about 30 minutes.
Meanwhile, in the 5-quart bowl of a professional-size 10-speed mixer fitted with the paddle attachment, beat together eggs, 6 egg yolks, and remaining 1/2 cup sugar on high speed. Slowly add canola oil and mix until pale and fluffy, 3 to 5 minutes. Add vanilla, lemon zest, and rum; mix to combine.
Attach the dough hook attachment to mixer. Add salt, 4 2/3 cups all-purpose flour, and yeast mixture. Mix on low speed until dough comes together. Slowly add butter; continue mixing until dough is no longer sticky, 10 to 15 minutes, adding remaining 2/3 cup all-purpose flour, a tablespoon at a time, to prevent sticking. Alternatively, you can knead dough by hand until smooth, 20 to 30 minutes. Transfer to a large lightly-oiled bowl; shape into a ball and lightly coat top with oil. Cover with a clean kitchen towel, followed by a layer of plastic wrap, and then a second clean kitchen towel; let stand in a warm place until doubled in size, about 2 hours.
Line the bottom of two 6-quart ovenproof saucepans with waxed paper rounds. Line sides with waxed paper, leaving an overhang of 3 inches; spray paper with nonstick cooking spray and set aside.
Weight out two 1 1/2-pound pieces of dough; pinch and shape each piece into a flat disk and transfer to prepared saucepans. Using a toothpick, pierce the dough to remove air bubbles. Cover with a clean kitchen towel; let stand in a warm place while preparing decorations.
Form remaining dough into suns, cross shapes, and fertility symbols for decoration. Brush tops of each loaf with egg white and top with decorative dough; secure using toothpicks. Cover with a clean kitchen towel and let loaves stand in a warm place until dough comes to top of pan, about 2 hours.
Preheat oven to 325 degrees. In a small bowl, whisk together remaining 2 egg yolks with 2 teaspoons water. Brush tops of loaves with egg yolk mixture and transfer to oven; bake until tops are golden brown, about 1 hour. If paska begins to brown too quickly, cover with parchment paper-lined aluminum foil.
Remove paska from oven. Immediately remove from pans and gently remove toothpicks from bread. Transfer to clean kitchen towel and cover with a second clean kitchen towel to cool. Paska can be frozen for up to 6 months.