In the bowl of a stand mixer with the whisk attachment, whisk the egg yolks, whole egg, sugar, butter and buttermilk.
Add approximately 280g of the flour along with the yeast and salt; whisk until moistened and combined. Remove the whisk attachment and replace with a dough hook.
Add all but 100g of the remaining flour and knead on low speed for 5 minutes. Check the consistency of the dough and add more flour if necessary; the dough should feel soft and moist but not sticky.
Knead on low speed 5 minutes more or until the dough clears the sides of the bowl.
Turn the dough out onto a lightly floured work surface; knead by hand about 30 seconds. Lightly oil a large bowl. Transfer the dough to the bowl, lightly oil the top of the dough, cover and let double in volume, 2 to 2 1/2 hours.
In a small saucepan over medium heat, combine the 225g of unsalted butter, brown sugar, rosemary, and sultanas.
Cook, stirring occasionally, until the butter is melted and the sugar is dissolved. Pour half of the topping into the bottom of 2 bundt pans and set aside.
Cover and store the other half of the topping in the refrigerator until the next morning.
Place the melted butter and rosemary for the coating in a medium shallow bowl and stir to combine. Once the dough has risen, turn out onto a lightly floured surface.
Portion the dough into 30g pieces; roll each piece into a ball. (You should have approximately 36 balls.) Roll the balls in the melted butter and rosemary.
Divide the balls evenly between the 2 bundt pans. Cover with cling film and place in the refrigerator overnight or up to 16 hours.
Remove the bread from the refrigerator and place in an oven that is turned off. Fill a shallow pan 2/3-full of boiling water and set on the rack below the bread.
Close the oven and let the bread rise until slightly puffy looking, 20 to 30 minutes. Once the bread has risen, remove it and the shallow pan of water from the oven.
Preheat the oven to 180 degrees C/Gas 4.
Once the oven is ready, place the bread on the middle rack and bake until slightly golden on top, approximately 25 to 30 minutes, or until the internal temperature reaches 90 degrees C on an instant-read thermometer.
Place the remaining topping in a small saucepan and set over medium heat. Reheat until the mixture is pourable, approximately 5 minutes.
Fifteen minutes into baking, pour the remaining topping over the bread, and finish cooking. Cool on a wire rack for 5 minutes, then invert onto a platter or cutting board. Serve immediately.