Step 1Heat the oven to 350 degrees.
Step 2Toast the bread: In a large bowl, toss the bread cubes with 2 tablespoons oil, the garlic powder, one-half teaspoon salt and a pinch of pepper. Place in a single layer on a baking sheet and toast until golden, 10 to 12 minutes, turning every few minutes.
Step 3Make the vinaigrette: In a medium bowl, whisk together the sherry vinegar with the shallot and mustard. Slowly whisk in the remaining three-fourths cup olive oil to emulsify. Season with one-eighth teaspoon salt, or to taste.
Step 4Onto each skewer, thread 2 toasted cubes of bread, 2 tomatoes, 2 mozzarella balls and 2 basil leaves. Just before serving, drizzle liberally with sherry vinaigrette.