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. If the yeast does not foam, it is dead (your dough will not rise) and you will need to start over with fresh yeast.
Add flour, sourdough discard, and salt and mix on low until combined and no dry spots remain. This will be a very shaggy, wet dough (you couldn't knead it if you tried!).
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 and the dough fills (or nearly fills) the pan.
Near the end of the rise, preheat the oven to 450°F 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. This will look a bit messy!
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. If you want a thinner frosting, add more milk. Drizzle the frosting over the focaccia, slice and serve!
Video
Notes
*You do not need a stand mixer to make this recipe. You can also make the dough in a large bowl with a wooden spoon. ** If doubling or tripling this recipe, bake each focaccia in its own 9x13-inch pan (instead of a larger pan). To make this recipe with active sourdough starter, omit the yeast and replace the discard with active starter (200g). You will also need to increase the rise times.