Soak Golden Raisins:
Put 2 cups golden raisins into a small bowl; add boiling water to submerge. Let sit for 15 minutes, then drain and rinse; set aside.Activate Yeast Mixture:
Warm 1 ½ cups whole milk in a small saucepan over high heat for roughly 30 seconds until lukewarm (barely warm). Pour into a small bowl; mix in ¼ cup sugar until dissolved. Scatter 1 tbsp active dry yeast on top of the milk-sugar mixture, stir, and leave in a warm spot for 15 minutes until bubbly and aromatic.Melt Unsalted Butter:
Melt 4 oz unsalted butter in a microwave or on the stovetop; allow to cool.Beat Eggs and Sugar:
Using a stand mixer bowl, beat 4 large eggs and 1 cup sugar on high speed for 10 minutes until frothy and light yellow.Combine Wet Ingredients:
Lower speed to low; gradually incorporate the yeast mixture. While still on low, add ½ tsp salt and 1 tsp vanilla extract. Next, pour in the cooled melted butter. Lastly, fold in the soaked raisins.Knead Dough:
Attach the dough hook. Gradually add flour, 1 cup at a time, until all 6 cups are used. Knead until dough is smooth and elastic, about 15 minutes.First Rise:
Cover the dough with a kitchen towel or plastic wrap; allow to rise in a warm location for 2 hours until tripled in volume.Divide and Shape Logs:
Turn dough out onto a floured surface; cut in half. Roll one half into a log, then cut into 4 equal portions. Roll each portion into a thin log about 1.5 feet long and 1 inch thick.Braid Challah Loaves:
Press the ends of the 4 logs together, braid them evenly, then press the opposite end together. Set on a greased baking sheet. Repeat with the remaining dough half.