2 1/4 tsp. active dry yeast
2 tsp. honey
5 cups (625 g) all-purpose flour
5 tsp. Diamond Crystal or 1 Tbsp. Morton kosher salt
6 Tbsp. extra-virgin olive oil, divided, plus more for hands
4 Tbsp. unsalted butter, plus more for pan
Flaky sea salt
2–4 garlic cloves
Whisk 2 1/4 tsp. active dry yeast, 2 tsp. honey, and 2 1/2 cups lukewarm water in a medium bowl and let sit 5 minutes.
Add 5 cups (625 g) all-purpose flour and 5 tsp. Diamond Crystal or 1 Tbsp. Morton kosher salt and mix with a rubber spatula until a shaggy dough forms and no dry streaks remain.
Pour 4 Tbsp. olive oil into a big bowl that will fit in your refrigerator. Transfer dough to bowl and turn to coat in oil. Cover with a silicone lid or plastic wrap and chill until dough is doubled in size, at least 8 hours and up to 1 day. If you're in a rush, you can also let it rise at room temperature until doubled in size, 3–4 hours.
Generously butter an 18x13" rimmed baking sheet. The butter may seem superfluous, but it’ll ensure that your focaccia doesn’t stick. Pour 1 Tbsp. extra-virgin olive oil into center of pan.
Keeping the dough in the bowl and using a fork in each hand, gather up edges of dough farthest from you and lift up and over into center of bowl. Give the bowl a quarter turn and repeat process. Do this 2 more times; you want to deflate dough while you form it into a rough ball.
Transfer dough to prepared pan. Pour any oil left in bowl over and turn dough to coat it in oil. Let rise, uncovered, in a dry, warm spot until doubled in size, at least 1 1/2 hours and up to 4 hours.
Place a rack in middle of oven; preheat to 450°. Lightly oil your hands. Gently stretch out dough to fill baking pan. Dimple focaccia all over with your fingers, like you’re aggressively playing the piano, creating very deep depressions in the dough (reach your fingers all the way to the bottom of the pan). Drizzle with remaining 1 Tbsp. extra-virgin olive oil and sprinkle with flaky sea salt. Bake focaccia until puffed and golden brown all over, 20–30 minutes.
Hold off on this last step until you're ready to serve the focaccia: Melt 4 Tbsp. unsalted butter in a small saucepan over medium heat. Remove from heat. Peel and grate in 2–4 garlic cloves with a Microplane (use 2 cloves if you’re garlic-shy or up to 4 if you love it). Return to medium heat and cook, stirring often, until garlic is just lightly toasted, 30–45 seconds. (Or, if you prefer raw garlic to toasted garlic, you can grate the garlic into the hot butter, off heat, then brush right away.)
Brush garlic-butter all over focaccia and slice into squares or rectangles.
Do Ahead: Focaccia is best eaten the day it's made, but keeps well in the freezer. Slice it into pieces, store it in a freezer-safe container, then reheat it on a baking sheet in a 300° F oven.