Whisk the sourdough discard, warm milk, 2 tablespoons sugar, and yeast in a large bowl. Let sit 5 minutes until foamy. Stir in the 3 tablespoons melted butter, egg, and salt, then mix in the flour to form a shaggy dough. Knead 8 minutes until smooth. Place in a greased bowl, cover, and refrigerate overnight.

Punch down the cold dough and divide into 40–50 pieces. Roll into balls, dip each in melted butter, and coat in the cinnamon sugar mixture.

Preheat oven to 350°F (175°C). Make the caramel by melting 3/4 cup butter with brown sugar, then stir in cream, vanilla, and salt. Pour into a greased Bundt pan. Layer the dough balls on top, drizzle with any extra butter and sugar.

Bake 30–35 minutes until deep golden. Cool 5 minutes, then invert onto a plate and spoon any remaining caramel over the top. Serve warm.