In a stand mixer, mix together the sourdough discard, warm milk, sugar, melted butter, and yeast until smooth and well combined.
Add the salt, then gradually mix in the flour, starting with 3½ cups. Stir until a soft, slightly sticky dough forms, adding more flour a little at a time if needed.
Knead for about 5 minutes, until it becomes smooth, elastic, and no longer shaggy. Place the dough in a lightly greased bowl, cover, and let rise in a warm spot for about 1 hour, or until doubled in size.
Generously oil a 9 x 13-inch baking dish. Gently press and stretch the dough into the pan, using your fingertips to create shallow dimples across the surface, just like focaccia.
In a small bowl, mix the brown sugar and cinnamon. Drizzle the melted butter evenly over the dough, then sprinkle the cinnamon sugar mixture over the top, making sure it reaches into the dimples.
Cover and let the dough rise again for about 20 minutes, until slightly puffy.
Bake in a preheated 375°F oven for 25–30 minutes, or until golden brown and baked through.
While the focaccia is still warm, whisk together the powdered sugar, milk, and vanilla. Drizzle the glaze generously over the top, letting it melt into all the cinnamon-swirled crevices before slicing and serving.