Use a sharp knife to cut, chunk, and grate enough Parmigiano-Reggiano to equal 1½ cups. *Note - If using a food processor, pulse to a semi-fine grate. If using a box grater, use the small grating holes.
Add the salt, pepper, cayenne pepper, and flour to the Parmigiano-Reggiano. Pulse to just combine. If using a bowl, stir to combine.
Add 1 stick of cold butter, cut into chunks, into the food processor. Pulse to blend the butter into the Parmigiano-Reggiano and flour mixture. If using a bowl, use a pastry blender or two forks to work the butter into the Parmesan and flour mixture.
Add the fennel seed and milk to the Parmesan and flour mixture. Pulse just until the mixture comes together into a dough that holds together in your hand when pressed. If using a bowl, use hands to bring the mixture together to form a soft dough.
Turn the dough out onto a lightly floured surface and press to form a mound.
Gently roll the dough into an approximate 15" x 12" rectangle, ¼" thick. *Note press the dough with your hands to make even edges if needed.
Use a pizza wheel, pasta wheel, or paring knife to cut the dough into approximately 1" square crackers.
Transfer the crackers to a parchment lined baking sheet and bake in a preheated oven for 14-16 minutes at 375-degrees, or until the crackers and golden brown and crispy.
Cool and serve. These crackers can be stored in an airtight container at room temperature for 3 days.