Soak the Raisins in the Dark Rum with a splash of Water and a teaspoon of Granulated Sugar; set aside for at least 30 minutes so they plump and absorb flavor.
Warm the Whole Milk until just lukewarm (about 100–110°F / 38–43°C), stir in a teaspoon of Granulated Sugar, then sprinkle the Active Dry Yeast over the surface; let sit 5–10 minutes until foamy.
In a large bowl combine the All-Purpose Flour with 1/2 teaspoon of Salt and 2 tablespoons of Granulated Sugar.
In a separate bowl whisk together the Egg Yolks, the Egg, Vanilla Extract and the Lemon Zest. Add the foamed milk-yeast mixture to the eggs and stir to combine.
Pour the wet ingredients into the dry ingredients and mix until a shaggy dough forms. Turn onto a lightly floured surface and knead while gradually adding softened Unsalted Butter in pieces, kneading until the dough is smooth, elastic and slightly tacky (about 8–10 minutes by hand or 5–6 minutes in a stand mixer).
Place the dough in a lightly oiled bowl, cover, and let rise in a warm place until doubled in size, about 1–1½ hours.
While the dough rises, make the filling: beat softened Butter with 1/2 cup Granulated Sugar until creamy, then fold in the rum-soaked Raisins (drain any excess liquid) and a little extra Lemon Zest if you like.
When the dough has doubled, punch it down and roll it out on a floured surface into a rectangle about 12x18 inches. Spread the raisin-butter filling evenly over the dough, leaving a 1/2-inch border.
Starting from the long edge, tightly roll the dough into a log. Trim the ends, then slice the log lengthwise down the middle to expose the layers. Twist the two halves together, braid them with the cut sides up, and transfer the braid (folded seam-side down) into a greased 9x5-inch loaf pan.
Cover the loaf and let it rise again until puffy, about 45–60 minutes. Meanwhile preheat the oven to 350°F (175°C).
Optional: whisk the remaining beaten Egg and brush lightly over the loaf before baking for a glossy finish. Bake the babka for 35–45 minutes, tenting with foil if the top browns too quickly, until deep golden and the center registers about 190°F (88°C).
Make the glaze while the babka bakes by whisking Powdered Sugar with enough Lemon Juice (and a teaspoon of Water if needed) to reach a thick but pourable consistency.
Remove the babka from the oven, brush generously with the lemon glaze, and let cool in the pan for 10–15 minutes before transferring to a wire rack to cool completely. Slice and serve.
