Dissolve yeast in 1/4 cup milk and set aside until frothy. Combine flour, sugar, salt and lemon zest in the bowl of a food processor and pulse a couple of times to combine. Add cubed butter and pulse until the mixture resembles bread crumbs (similar texture to a pastry dough). Add in yeast mixture and then egg yolks and pulse again until just combined. Add remaining milk a few tablespoons at a time until dough just starts to come together. You could also do this process by hand: combine the dry ingredients as above in a large bowl and blend in the butter cubes by rubbing it in with your fingers until it resembles bread crumbs. Make a well in the center and add in the yeast mixture and then the eggs. Mix with a fork and add in more milk as needed until dough just starts to come together. Turn out onto a floured surface and knead for 3-4 minutes until smooth and elastic but not sticky. Divide into 4 pieces, wrap each in plastic wrap and chill for 30 minutes.