I am obsessed with my panini maker. I believe everyone should own one. Many of the new versions have two-sided cooking surfaces, like this one, that allow you to use them as a griddle when you’re not grilling pressed sandwiches. This panini is unique because it features focaccia, a dense, “quilty” Italian bread whose spongy texture soaks up flavor. I also like the fact that focaccia doesn’t have an overly chewy crust — it’s perfect for your picnic enjoyment!
Slice the focaccia in half lengthwise, and then cut each half into quarters, forming four tops and bottoms. (If you prefer smaller sandwiches, cut accordingly.) Using a pastry brush, coat the inside of the top and bottom layers with the olive oil. Brush the balsamic vinegar on each slice in the same way.
Arrange the bottom slices on a tray and layer the ingredients on them one at a time. Start with the mozzarella and then top with the tomato, the prosciutto, and the arugula. Season with salt and pepper to taste and drizzle with a little more vinegar, if you like. Add the top slices to close the sandwiches, and grill on your panini press until the mozzarella is melted, about 2 minutes. If you don’t have a panini press, place the paninis on a grill pan or griddle and press them with the bottom of a heavy skillet covered in aluminum foil. Serve warm or at room temperature.