In a small bowl, combine warm milk, yeast, and 1 tsp sugar. Let it sit 5–10 minutes until foamy.

In a large bowl, whisk together flour, remaining sugar, and salt. Add eggs, melted butter, vanilla extract, and yeast mixture. Mix until a sticky dough forms.

Knead the dough on a floured surface for 8–10 minutes until smooth and elastic.

Place dough in a greased bowl, cover, and let rise for 1–1.5 hours until doubled.

Punch down dough, spread evenly in a lined 9×13-inch pan.

Drop jam spoonfuls across dough and swirl lightly. Dot with softened butter.

Cover and let dough rise 30–40 minutes.

Preheat oven to 350°F (175°C) and bake for 25–30 minutes until golden brown.

Cool 10–15 minutes, dust with powdered sugar, add berries if desired, slice, and serve.