In a small bowl, combine 2/3 cup of warm milk (about 110°F) and the package of dry active yeast. Let it sit for 5-10 minutes until frothy.
In a large mixing bowl, whisk together 2 2/3 cups of flour, 1/3 cup sugar, and 1/4 teaspoon salt. Create a well in the center and add the activated yeast mixture, 1 egg, and 1/3 cup melted butter. Mix until dough begins to form.
Transfer dough to a lightly floured surface and knead for 5-7 minutes until smooth and elastic. Add more flour if dough is sticky.
Place dough in a greased bowl, cover with a clean towel, and let rise in a warm place for 1 hour or until doubled in size.
While dough rises, prepare raspberry filling by rinsing fresh raspberries or thawing frozen raspberries and draining excess liquid. Optionally sweeten with 1 tablespoon sugar.
Punch down risen dough and roll out on floured surface into a large rectangle about 1/4 inch thick. Cut slits along sides leaving center intact for filling.
Spread raspberries evenly down the center of the dough rectangle, being careful not to overfill.
Braid the Danish by folding strips of dough over the filling, alternating sides. Press edges gently to seal.
Transfer braided Danish to a parchment-lined baking sheet, cover, and let rise for another 30 minutes.
Preheat oven to 375°F. Whisk together remaining egg yolk and 1 teaspoon water, then brush over Danish for golden finish.
Bake Danish for 20-25 minutes until golden brown and aromatic.
Cool on wire rack for a few minutes before slicing. Serve warm or at room temperature.