In a large bowl, mix warm milk, sugar, and yeast. Let sit for 5 minutes until foamy.
Add melted butter, egg, vanilla extract, and salt. Mix thoroughly.
Gradually add flour and mix until a soft dough forms.
Knead dough for 6 to 8 minutes until smooth and elastic.
Place dough in a greased bowl, cover, and let rise until doubled in size, about 1 hour.
Grease a 9x13-inch baking pan and press dough gently into it.
Use your fingers to poke dimples evenly across the surface of the dough.
Spoon small dollops of jam into the dimples and swirl lightly with a toothpick.
Cover and allow the dough to rest for 20 minutes for a second rise.
Preheat the oven to 350°F (175°C) and bake the focaccia for 22 to 25 minutes until golden.
Brush the hot focaccia with melted butter and sprinkle generously with sugar. Optionally dust with powdered sugar.
Slice and serve warm, perfect alongside coffee or milk.