Heat the oven to 425°F and arrange a rack in the middle.

Place the pancetta on a rimmed baking sheet in an even layer and bake until browned and crisped, about 15 minutes.

Transfer to a paper-towel-lined plate to drain and set aside.

Place the eggs, milk, and measured salt and pepper in a large bowl and whisk until the eggs are broken up and the mixture is thoroughly combined; set aside.

Melt 2 tablespoons of the butter in a large nonstick frying pan over medium-low heat until foaming.

Pour in the egg mixture and let it sit undisturbed until the eggs just start to set around the edges, about 1 to 2 minutes.

Using a rubber spatula, push the eggs from the edges into the center.

Let sit again until the edges start to set, then spread back into an even layer. Repeat, pushing the eggs from the edges into the center every 30 seconds, until almost set, for a total cooking time of about 4 to 5 minutes.

Meanwhile, toast the bread and spread the remaining 2 tablespoons of butter on it.

Remove the pan from the heat and gently stir the pesto into the eggs.

Divide the eggs among half of the bread slices. Divide the pancetta over the eggs and top with the remaining bread slices butter-side down.

Serve immediately.