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1
Heat the oven to 375 degrees F and arrange a rack in the middle.
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2
Bring a large saucepan (about 4 quarts) of heavily salted water to a boil over high heat.
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3
Add the pasta and cook according to the package directions.
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4
When the pasta is al dente, reserve 1/4 cup of the cooking water, then drain.
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5
Meanwhile, cut the prosciutto in half crosswise.
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6
Place 1 slice in each well of a 12-well muffin pan, pressing the prosciutto evenly into the bottom and up the sides of each well; set aside.
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7
Melt 1 tablespoon of the butter in a small frying pan over medium heat until foaming.
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8
Add the panko and cook, stirring occasionally, until light golden brown, about 4 minutes.
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9
Remove the pan from the heat and set aside.
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10
While the pasta is draining, rinse the saucepan and wipe it dry.
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11
Melt the remaining 2 tablespoons of butter in the saucepan over medium-low heat until foaming.
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12
Add the flour and whisk until smooth.
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13
Cook, whisking occasionally, until the flour has darkened slightly in color, about 2 to 3 minutes.
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14
Add the milk very slowly, whisking it into the flour-butter mixture.
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15
Continue cooking, whisking constantly to smooth out any lumps, until the mixture simmers and thickens, about 6 to 7 minutes.
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16
Remove the saucepan from the heat.
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17
Add the reserved pasta water, cheeses, mustard, measured salt, and pepper and whisk until the cheese has melted and the mixture is smooth.
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18
Add the drained pasta and stir to combine.
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19
Divide the pasta mixture among the wells of the prepared muffin pan and sprinkle with the toasted panko.
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20
Cook until the edges of the mac n cheese cups are bubbling slightly and the tops are light golden brown, about 25 to 30 minutes.
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21
Remove the muffin pan to a wire rack and let it cool for 5 minutes.
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22
Run a small knife around the perimeter of each well to loosen and remove the prosciutto cups.
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23
Serve immediately.