Overnight Prep: Whisk together yeast, sugar, and warm water in a large bowl. Let sit for 5 minutes until mixture is foamy and fragrant. *
Add in flour and salt. Mix together by hand until shaggy dough forms and there are no more dry streaks of flour.
Grab a large bowl that will fit in your fridge.* Pour 3 tablespoons oil into bowl. Transfer dough into oiled bowl. Cover with lid or plastic wrap. Keep in the fridge for at least 8hours, up to 24 hours.
Prepare Cinnamon Sugar Filling: Whisk together brown sugar and cinnamon until well blended. Set aside until ready to use.
Generously butter 13x9-inch baking pan. Pour remaining one tablespoon oil into center of pan. Transfer chilled dough to prepared pan. Sprinkle half of cinnamon sugar mixture over dough.
Use your hands to gather the edges of the dough, lift up and bring edges to center of dough to cover up the filling.
Sprinkle remaining half of cinnamon sugar over dough. Repeat lifting edges of the dough toward the center to cover filling.
Turn down over to reveal smooth surface bottom. Pour any oil leftover in bowl (used for resting in the fridge) over dough.
Let dough rise, uncovered, in a warm area until doubled in size and dough covers most of the pan surface, about 90 minutes to 4 hours (depending on ambient temperature). Make crumb topping while dough rises.
Prepare Crumb Topping: In a medium bowl, whisk or stir together brown sugar, flour, cinnamon, and salt. Cut in cold butter with a pastry blender or two forks, until mixture forms large clumps and crumbles. Keep chilled until ready to use.
Place rack in center of oven. Preheat oven to 400℉. Lightly oil or butter your fingers. Dimple dough all over with your fingers, making sure you touch the bottom of the pan.
Sprinkle crumb topping all over dough surface. Bake focaccia until golden brown and fragrant, about 20-30 minutes. Rotate pan midway through baking to ensure even cooking. If crumble topping is browning too much, lightly over pan with foil during the last 10-15 minutes of baking.
Slicing and Serving: Cool focaccia in pan for about 5 minutes. Unmold onto wire rack to cool further, if desired. Place warm focaccia on cutting bowl. Use a serrated bread knife to gently slice through bread in sawing motion. Be careful not to press down forcefully or crush the bread.
Cinnamon focaccia is best served warm, the same day it is baked. Keep leftovers in a covered container up for two days. Reheat on baking sheet in 300-325°F oven until warm and toasty.