Mix flour, salt, yeast, olive oil and softened butter together in a bowl; add just enough hot milk to form a soft dough. Cover the bowl with a clean tea towel; let it rise until doubled in volume, about 1 hour.
Punch down the dough; turn out onto a lightly floured surface and knead for about 10 minutes or until smooth and elastic. Set dough aside in bowl to rise a second time, at least 1 hour.
Preheat the oven to 200 C / Gas 6. Lightly grease a deep-sided baking tray.
Transfer the bread dough into prepared baking tray; scatter chopped sage on top. Dip your fingers into a bowl of warm water; press into the foccacia dough to make deep dents all over the top, wetting your fingers each time, to leave the top of the loaf moist. Brush the dough with olive oil and sprinkle with coarse salt.
Bake bread in preheated oven for 30 minutes; serve warm.