Step 1

In a large pot of boiling salted water, cook macaroni until al dente according to package instructions; drain.

Step 2

In pot, melt 4 tablespoons butter over medium heat. Whisk in flour, and cook, stirring frequently, until mixture is pale golden and has a nutty aroma, about 4 minutes. Whisking constantly, slowly add milk. Cook, whisking along bottom of pot, until boiling, about 7 minutes. Reduce heat and gently simmer, stirring occasionally, until sauce is thickened, 10 to 12 minutes. Remove from heat, and add remaining 1 tablespoon butter, the mustard powder, and cheese. Stir until melted and sauce is smooth; season with salt. Add macaroni to sauce, and stir to combine.

Step 3

Line a 9-by-13-inch baking dish with parchment; brush parchment lightly with oil. Spread macaroni mixture evenly in dish. Cover and refrigerate until firm, at least 8 hours.

Step 4

Invert set macaroni and cheese onto a cutting board and remove parchment. Cut into 1-inch squares with a sharp knife.

Step 5

Whisk eggs in a shallow dish. In a shallow dish, season panko with salt. Dip macaroni squares, a few at a time, in eggs, allowing excess to drip off into dish, then roll in panko mixture to coat completely. Transfer to a wire rack set on a baking sheet.

Step 6

Heat 2 tablespoons oil in a large nonstick skillet over medium until shimmering. Working in batches, add coated squares in a single layer (do not crowd pan); cook, turning occasionally, until all sides are crisp and golden brown, 5 to 7 minutes total. Transfer to a paper-towel-lined rack to drain and cool slightly. Wipe skillet clean. Repeat process with more oil and remaining squares. Serve warm.