Slice the bread into 1-inch cubes then arrange it on a large baking pan and let it get stale for at least 2 hours or overnight, tossing a few times to make sure it all gets stale.
Whisk the eggs together in a large bowl then whisk in the milk, sugar, brown sugar, cinnamon, vanilla and salt.
Gently toss in the bread, cover and refrigerate for at least 4 hours or up to 8 hours.
Pour the mix into a crockpot then place the pieces of butter around on top.
Cover and cook on high for 1 ½-2 hours or on low for 4 hours or until the eggs are set.
Make the glaze by whisking the powdered sugar and cinnamon together in a medium bowl then pour in the milk and vanilla and stir to combine. Whisk in more milk as needed.
Drizzle the glaze on top and serve.