1. Add the bread flour, chilled butter, baking powder, sugar, and salt to a food processor and pulse a few times, until the butter pieces are about the size of peas. If you don't have a food processor, you can gently work the dough together with your hands or use a pastry cutter.
2. Add in the water and pulse a few more times to incorporate. Don't overwork the dough.
3. Shape and chill your dough. Move the dough to a baking mat or another clean surface and press it together until it forms a ball. Flatten it into a ½ inch thick rectangle, wrap with plastic wrap, and chill in the fridge for at least an hour.
4. Meanwhile, preheat your oven to 350˚F and line a baking sheet with parchment paper.
5. While the dough is chilling, combine cheddar cheese and milk in a small saucepan over low heat, until the cheese is melted. Whisk or stir in the corn starch until the cheese starts to thicken, then transfer to a small bowl and place in the refrigerator.
6. Roll your chilled dough into a rectangle about ⅛-inch thick. Cut into rounds with a 1-inch fluted pastry ring or cookie cutter and carefully transfer to the baking sheet. The crackers don't really expand, so you can space them closely together.
7. Using a fork, pierce the crackers 2-3x each (this keeps them from puffing up too much in the oven). In a small bowl, whisk together the egg and water. Brush each cracker lightly with egg wash and sprinkle with salt.
8. Bake for about 15 minutes, until the crackers are crisp and brown on the bottom. Let them cool.
9. While the crackers bake, break apart the cooled cheese mixture into chunks and pulse in the food processor for about 10 seconds, until smooth. Transfer the cheese mixture to a piping bag or a plastic bag and cut off the tip.
10. Pipe about 1/2 teaspoon of cheese filling between two crackers and sandwich together. Repeat with all crackers. Enjoy!

Storage: Because of the cheese filling, these cracker sandwiches are unfortunately not shelf stable and will need to be refrigerated. The crackers stay most crisp at room temperature, so if you would prefer, you can keep the crackers stored separately. Keep the cheese filling in a container in the fridge for up to a week and fill just before eating.