In a small bowl, combine warm water with sugar and whisk until dissolved. Sprinkle in active dry yeast and let it sit for about 5 minutes until bubbly.

In a stand mixer, combine bread flour, all-purpose flour, and salt. Slowly add the yeast mixture until a sticky dough forms. Knead on low for 5 minutes.

Transfer the dough to a greased bowl, cover, and let rise in a warm place for about 1 hour until doubled.

Grease a 9x13-inch baking pan and gently punch down the dough, transferring it to the pan and stretching it into the corners.

Cover the pan and let the dough rise for another 1 hour until puffy.

Preheat oven to 425°F (220°C).

Poke dimples in the dough for the preserves, spoon raspberry jam into these dimples, and bake for 18–22 minutes until golden.

After baking, poke more holes and add additional raspberry preserves while warm.

Whisk together confectioners' sugar and whole milk to make a glaze, then pour it over the focaccia.

Cut into squares and serve warm or at room temperature.