Peel and dice apples into small pieces. Set aside.
In a medium saucepan, melt butter over low heat. Remove from heat, let cool slightly, and add brown sugar, cinnamon, and vanilla extract. Be careful to remove from heat so the sugar doesn't burn. Stir to combine and fold in apples.
Scoop about 1/3 of the cinnamon sugar apple mixture onto the dough in the pan. Fold it into the dough by pulling the sides of the dough into the center. It will be very messy and that's okay! Flip the dough over so it is seam side down in the pan. Spread the dough back out by pushing it gently to the edges with your fingers.
Cover pan with damp kitchen towel. Place pan back in oven with the oven light on for a 2nd rise for 1 hour or until the dough has nearly doubled in size and mostly filled pan. The dough will be bubbly and will have expanded to nearly reach the edges. *See Note about using warmth to speed up rise times.
After 2nd proof, remove baking pan from oven and place on counter. Preheat the oven to 425℉ while you add the rest of the topping.
Dimple Dough: On low heat, warm the remaining cinnamon sugar apple mixture up on the stove to make it pourable. Scoop the remaining mixture all over the top of the dough. It may look soppy and runny. Use your fingers to poke the dough to create "dimples." Gently push the dough all over to expand it fully in the pan and push down the apple fritter mixture. The dough should be dimpled and covered with topping all over.
Bake Focaccia: Place pan on the lowest rack of your oven. Bake for 25-30 minutes until focaccia is bubbling and golden brown.