How to Make It
Preheat the oven to 350°. Brush the brioche slices with 2 tablespoons of the olive oil and arrange them on a baking sheet. Toast for about 5 minutes, until they are golden and crisp. Let cool, then break the toasts into bite-size pieces.
Using a paring knife, make a small slit in each date and remove the pit. Fill a small sturdy plastic bag with the mascarpone and snip a bit from one corner. Pipe the mascarpone into the dates and place them on a baking sheet. Bake for about 5 minutes, just until the mascarpone melts and the dates are warmed through.
In a medium bowl, whisk the balsamic vinegar with the remaining 2 tablespoons of olive oil and season with salt and pepper. Add the watercress and toss to coat. Add the brioche and toss gently. Arrange the salad on plates, garnish with the dates and serve right away.