Preheat oven to 375°. Toast and cool, or leave bread cubes out overnight to allow them to dry out. Lightly grease four ramekins with butter. Divide the bread cubes between them. Combine milk, brown sugar, cinnamon, vanilla and eggs in a bowl or measuring cup. Pour the custard over the bread but leave a little space at the top. Gently press the cubes down into the custard to submerge and let them rest for about 10 minutes to get fully saturated. Place on a sheet pan and bake for 30 minutes.