Preheat the oven to 375ºF. Butter the bottom and sides of four 8-oz. ramekins or mini cocottes. Alternatively, use an 8- or 9-inch pie pan.
Whisk together eggs, milk, salt, and pepper, until completely blended.
Add a 1/3 cup layer of cubed bread to the bottom of each ramekin. Cover with dollops of mascarpone and pesto, totaling 1 tablespoon of each. Add another 1/3 cup layer of cubed bread, and more dollops of mascarpone and pesto, totaling another 1 tablespoon of each. (Each ramekin gets 2/3 cup bread cubes, 2 tablespoons of mascarpone, and 2 tablespoons of pesto total.)
Place ramekins on a large baking sheet. Pour about 3/4 cup of the egg mixture into each ramekin, filling to just below the top.
Bake for 30-35 minutes, until the strata has set and is beginning to brown on top. You may get some cheese and pesto spillover—that's what the baking sheet is for.
Let cool for 5 minutes, then top with a final dollop of mascarpone and pesto, garnish with fresh mint leaves, and serve.