In the bowl of a stand mixer fitted with a dough hook, combine the warm water and granulated sugar. Sprinkle yeast on top and let sit for 5 minutes until foamy.
Add flour, sourdough discard, and salt and mix on low until combined and no dry spots remain.
Transfer the dough to a large, oiled bowl and cover with plastic wrap. Let rise for 2-3 hours, until doubled in size.
Brush a 13x9-inch pan with 2 Tablespoons melted butter and set aside.
Once the dough has risen, use a spatula to scrape around the perimeter of the bowl, deflating the dough. Transfer the dough to the center of the prepared 13x9-inch pan. Cover with plastic wrap or a clean kitchen towel and let rise 1 ½-2 hours, until doubled in size.
Near the end of the rise, preheat the oven to 450°F (230℃) and make the topping. Melt the butter in a small saucepan over low heat. Remove from heat and stir in the brown sugar and cinnamon.
With wet fingers, press deep dimples all over the dough. Drizzle the dough evenly with the butter and brown sugar mixture.
Bake for 25-30 minutes, until the focaccia is golden and crusty. Let cool for 10 minutes in the pan, then transfer to a cooling rack to cool fully.
Combine powdered sugar, milk and vanilla extract in a small bowl. Whisk until smooth and combined. Drizzle the frosting over the focaccia, slice and serve!
