Activate the yeast: Combine the lukewarm milk, yeast, and granulated sugar. Stir and let stand for about 10 minutes, until foamy.
Mix the dough: Add the egg and vanilla. Gradually mix in the flour and salt until a soft dough forms.
Add the butter and knead: Knead in the softened butter a little at a time. Continue kneading for about 10 minutes, until smooth and elastic. The dough should remain soft and slightly tacky.
First rise: Shape the dough into a ball, place in a bowl, and cover with plastic wrap. Let rise for about 60 minutes, or until doubled in size.
Add the chocolate: Gently deflate the dough and transfer to a lightly floured surface. Roll into a rectangle and scatter with chopped chocolate. Fold the dough over the chocolate and gently knead just enough to distribute the pieces.
Shape and proof: Grease a loaf pan with butter. Shape the dough into a loaf and place seam-side down in the pan. Cover loosely and let rise for about 30 minutes, until noticeably puffy.
Bake: Toward the end of the second rise, preheat the oven to 340°F (170°C). Bake for about 30 minutes, until golden brown and baked through. Loosely cover with foil if the top browns too quickly.
Finish and cool: Immediately brush the hot loaf with a little butter. Let rest briefly in the pan, then carefully transfer to a wire rack. Cool until only slightly warm before cutting into 10 slices.