1. Sprinkle yeast over warm water in a large bowl. Let stand until foamy, about 5 minutes. Whisk in 2 teaspoons malt syrup until smooth. Stir in flour, coarse salt, buttermilk, and 3 tablespoons butter until combined. Transfer dough to a lightly floured work surface. Knead, dusting hands and surface with more flour as needed, until smooth and elastic, about 5 minutes. Shape into a ball.

2. Transfer dough to a large bowl brushed with butter, turning to coat. Cover and let rise in a warm spot until doubled in bulk, about 1 hour.

3. Preheat oven to 425 degrees. Punch down dough. Transfer to an 8-inch square baking dish brushed with butter, pressing to flatten evenly in bottom. Lightly dust with flour. Using a sharp knife, score dough almost to bottom of pan from corner to corner diagonally in an X, then score strips of dough diagonally in one direction in 2-inch intervals. Score in other direction to create 2-inch diamonds. Cover and let rise again until doubled in bulk, about 45 minutes.

4. Whisk together remaining 1 tablespoon each malt syrup and butter in a small bowl; brush over top of dough. Bake until puffed and golden brown, 15 to 18 minutes. (If tops are browning too quickly, tent with foil.) Sprinkle with flaky salt. Serve warm.