Warm the milk to about 105-115°F (lukewarm). In a large mixing bowl or the bowl of a stand mixer, whisk together 2 cups of the flour, instant yeast, granulated sugar, salt, and 1.5 tsp ground cinnamon until evenly distributed.

Hydrate and mix

Add the warm milk, warm water (about 105-115°F), softened butter (reserve 1/2 tablespoon for brushing later if desired), egg, and vanilla to the flour mixture. Mix with a dough hook on low-medium speed or by hand until combined, about 2-3 minutes.

Add remaining flour and knead

Gradually add the remaining 2 cups of flour, 1/2 cup at a time, mixing and then kneading until a soft, slightly tacky but not sticky dough forms. Knead in the mixer for 6-8 minutes on medium speed or by hand on a lightly floured surface for 8-10 minutes. Dough should be smooth and elastic.

First rise

Shape the dough into a ball, place it in a lightly oiled bowl, cover with plastic wrap or a damp towel, and let rise in a warm draft-free place until doubled in size, about 60-90 minutes depending on room temperature.

Raisin Mixture

Plump raisins

While the dough is rising, place the raisins in a small bowl and pour 1/4 cup warm water over them. Let soak 10-15 minutes to plump, then drain any excess liquid. If using, stir in the lemon zest.

Prepare filling

In a small bowl combine the plumped raisins, 2 tablespoons dark brown sugar, and 1 tsp ground cinnamon. Set aside until ready to assemble.

Shaping and Second Rise

Deflate and roll

After the dough has doubled, gently punch it down and turn it out onto a lightly floured surface. Roll the dough into a rectangle roughly 12x8 inches (30x20 cm).

Distribute raisin filling

Evenly sprinkle the raisin-sugar-cinnamon mixture over the dough rectangle, leaving a 1/2-inch border on one long edge. If any raisins remain slightly wet, pat them dry before sprinkling so they don't create soggy spots.

Roll into loaf

Starting from the long edge opposite the border, tightly roll the dough into a log. Pinch the seam and the ends to seal so the filling doesn't leak out. Place the seam-side down into a lightly greased 9x5-inch (23x13 cm) loaf pan.

Second rise

Cover the pan loosely with lightly greased plastic wrap or a towel and let the loaf rise until puffy and nearly doubled, about 45-60 minutes. Preheat the oven to 350°F (175°C) during the last 20 minutes of rising.

Bake and Finish

Bake

Optional: brush the top with the reserved 1/2 tablespoon softened butter for a softer crust. Bake the loaf in the preheated 350°F (175°C) oven for 30-35 minutes, rotating the pan halfway through, until the crust is golden brown and an instant-read thermometer inserted into the center reads 190-200°F (88-93°C). If the top browns too quickly, tent loosely with foil.

Cool

Remove the loaf from the oven and let it cool in the pan for 10 minutes. Then carefully lift the bread onto a wire rack and cool completely, about 1-1.5 hours, before slicing to ensure best texture.

Optional glaze

Whisk together powdered sugar, 1 tablespoon milk (or water), and 1/4 tsp vanilla until smooth. Drizzle over the cooled loaf. Let glaze set for 10-15 minutes before slicing.