Grease a 10-inch round Bundt or springform pan with 2 tablespoons butter.

Make the yeast mixture: Whisk together the yeast and ¾ cup milk in a small bowl until dissolved. Stir in 1 teaspoon flour and ½ teaspoon sugar. Cover the bowl and let the yeast activate for 5 to 10 minutes, until foamy air bubbles appear on the surface.

Make the dough: Mix the remaining 3 ½ cups flour, egg yolks, vanilla, remaining ¾ cup milk and the rested yeast mixture in a large bowl until it forms a very soft, light dough. Cover and let it proof for 30 to 45 minutes, until the dough has risen slightly (it will not double in size.)

Mix the remaining 10 tablespoons softened butter and ¾ cup sugar together in a medium bowl until smooth. In a small bowl, mix together the walnuts and cinnamon. Turn out the dough onto a well-floured surface (it will be very sticky). Roll the dough into approximately a 12- by 18-inch, 1/4-inch-thick rectangle. Spread the butter mixture over the surface of the dough with a spatula, then sprinkle over the walnut mixture.

Working from the longer edge, tightly roll the dough into a spiral. Cut into 1½-inch-thick slices. Place each roll into the cake pan, spiral-side up. Repeat with remaining slices of dough in a single layer in the cake pan. If necessary, start a second layer on top of the first. Cover and let rise for 1 to 2 hours, or until the rolls have doubled in size.

Preheat the oven to 350ºF.

Bake until golden brown, 40 to 50 minutes (rolls will bake longer in a Bundt than a springform pan). Remove from the oven and immediately flip the pan over onto a serving plate. Serve immediately, cutting or breaking apart while the rolls are still warm.