01 - Preheat oven to 400°F and line a baking sheet with parchment paper.
02 - Scrub potatoes, pat dry, and pierce several times with a fork. Rub with olive oil, kosher salt, and black pepper. Arrange potatoes on the prepared sheet and bake for 50 to 60 minutes, or until fork-tender. Allow to cool for 10 minutes.
03 - While the potatoes bake, cook elbow macaroni in a pot of salted boiling water until just al dente. Drain well and set aside.
04 - Melt butter in a saucepan over medium heat. Whisk in flour and cook for 1 minute. Gradually pour in whole milk, whisking continuously, until mixture thickens, about 3 minutes.
05 - Remove saucepan from heat. Stir in shredded cheddar, Parmesan, Dijon mustard, smoked paprika, and season with salt and pepper. Add the drained macaroni and fold until well coated.
06 - Slice cooled potatoes in half lengthwise. Scoop out the centers, leaving a 1/4-inch shell. Reserve scooped potato for another use.
07 - If using panko, combine with melted butter and toast in a small skillet over medium heat until golden.
08 - Fill each potato shell generously with the mac and cheese mixture. Sprinkle toasted panko on top of the filling, if desired.
09 - Return filled potato skins to the oven and bake for 10 minutes, until the cheese is bubbling and tops are golden brown.
10 - Remove from oven and garnish with chopped chives before serving.