Combine warm milk, sugar, and yeast in a large bowl; let sit for 5 minutes until foamy.
Add melted butter, egg, vanilla extract, and salt to the yeast mixture; mix well.
Gradually add flour to the wet ingredients until a soft dough forms.
Knead the dough on a floured surface for 6 to 8 minutes until smooth and elastic.
Place dough in a greased bowl, cover, and let rise for 1 hour until doubled in size.
Grease a 9x13-inch baking pan and gently press the dough into it.
Poke dimples across the dough surface using your fingers.
Spoon small dollops of jam into the dimples and swirl lightly with a toothpick.
Cover and let the focaccia rest for 20 minutes to rise again.
Preheat oven to 350°F (175°C) and bake the focaccia for 22 to 25 minutes until golden.
Brush hot focaccia with melted butter and sprinkle generously with sugar and optional powdered sugar.
Slice and serve warm, ideal with coffee or milk.