Preheat oven to 375 degrees F. Line a baking sheet with a silicone baking mat or parchment paper.
Combine milk, water, butter, sugar and salt In a heavy bottomed medium saucepan and heat on medium high.
Once the mixture is at a rolling boil, add all of the flour at once, reduce the heat to medium and start to stir the mixture vigorously with a wooden spoon. The dough should come together into a ball.
Continue stirring for another 3-4 minutes until it is completely smooth and soft.
Transfer the dough into a bowl of a stand mixer fitted with the paddle attachment and beat to cool it down slightly. Add in four eggs one at a time, beating after each egg has been added to incorporate it into the dough. The dough should be very thick and shiny but not liquidy.
Transfer the dough to a pastry bag fitted with a 1 inch plain tip. Pipe out mounds of dough about 2 inches wide and inch high on the prepared baking sheet, spacing about 2 inches apart.
Take out the craquelin dough from the refrigerator. Using a round cookie cutter about the same diameter of the puffs, cut out circles of the dough and set them on top of the puffs.
Bake in oven for about 40 minutes, rotating halfway through. The puffs should turn golden brown and be dry to the touch. It's easier to underbake than to overbake; make sure they are dry before taking out.
Place sheet on a wire rack and let puffs cool before slicing in half. Pull out any underbaked bits still inside.