In a small bowl, combine about 1/2 cup of warm milk (around 110°F), active dry yeast, and granulated sugar. Stir gently until dissolved, then let it sit for 5-10 minutes until foam forms.

In a large mixing bowl, whisk together all-purpose flour and salt. Add the yeast mixture, softened butter, eggs, vanilla extract, and optional lemon zest. Mix until a shaggy dough forms, then knead on a floured surface for 10-12 minutes until smooth and elastic.

Place the kneaded dough in a greased bowl, cover with a kitchen towel, and set in a warm place to rise until doubled in size, approximately 1 to 1.5 hours.

In a saucepan, mix raspberry preserves, cornstarch, and water over medium heat. Stir continuously until thickened, about 3-5 minutes. Remove from heat and let cool.

Once the dough has risen, punch it down gently. Roll it out into a rectangle about 1/2 inch thick. Spread the cooled raspberry filling evenly on top.

Slice the dough lengthwise into two halves, exposing the swirls. Twist the two halves together and place in a greased loaf pan. Cover and let it rise again until puffy, about 30-45 minutes.

Preheat the oven to 350°F (175°C). Brush the top of the loaf with melted butter before baking.

Bake the loaf for 35-40 minutes, until golden brown. Check with a toothpick; it should come out clean.

Allow to cool in the pan for 10 minutes, then transfer to a wire rack. Mix powdered sugar, lemon juice, and a splash of milk for the glaze, and drizzle over the warm loaf.