In a measuring cup measure out 2½ c. lukewarm water and honey. Add yeast and allow to bloom approx 5 minutes.
Add liquid to a large bowl. Add 5 cups flour and 1 Tbsp salt and mix with rubber spatula until the dough is cohesive and no dryness remains.
Oil a large bowl with 4 Tbsp olive oil. You’ll want a bowl large enough to handle the rising. Cover with plastic wrap and refrigerate 8 hours or overnight until doubled in size.
Using the Tbsp of butter, generously grease a 9" x 13" baking pan (best for sandwiches) or an 13” x 18" rimmed baking sheet (best for thin, snacking).
Remove the dough from the fridge. Using a fork in each hand, gently fold the dough in half 3 times. Allow to rise 1½ - 4 hours uncovered in a warm, dry spot.
Stretch the dough into the pan, then coat your hands in olive oil and dimple the focaccia all over with your fingers and drizzle with remaining olive oil. Sprinkle with flaky sea salt and bake 20-30 minutes until golden all over.