Preheat oven to 400 degrees.
In large pot of salted boiling water, cook macaroni pasta according to package instructions and drain.
In medium pot, whisk together butter and flour. Slowly whisk in milk and cook on medium-low until mixture thickens enough to coat the back of a spoon.
Reduce heat to low. Add shredded cheddar and cream cheese, continuing to whisk until incorporated and smooth. Mix in egg yolk. Remove from heat.
Butter the bottom and sides of muffin tins and sprinkle half of the Parmigiano Reggiano around edges to coat completely. Divide mac and cheese into each cup and top with the remaining Parmigiano Reggiano.
Bake approximately 15 minutes and allow to cool 5 minutes before removing from tins and serving.