In a small food processor or blender combine 1 cup fresh basil, 2 tsp toasted pine nuts, 1 tbs grated Parmesan, 1/2 tsp minced garlic, 1 tsp lemon juice, 1/4 tsp salt, and 1/8 tsp black pepper. Pulse to chop ingredients, then drizzle in 3 tbs olive oil while pulsing until a coarse, spreadable paste forms. Taste and adjust salt, pepper, or lemon as needed. If the pesto is too thick, add up to 1 tbs more olive oil.

Assemble & Toast Sandwiches

Prepare bread

Spread one side of each of the 4 bread slices lightly with softened unsalted butter. This will be the outside for toasting.

Add pesto

Flip the bread slices so the unbuttered sides face up. Evenly spread about 1 1/2 tbs of the prepared pesto on each of two unbuttered slices (about 3 tbs of pesto total).

Layer Caprese ingredients

On the two pesto-topped slices, layer sliced fresh mozzarella (about 4 oz per sandwich half), then 3-4 thin tomato slices per sandwich, then 4 fresh basil leaves. Lightly season the tomato with a pinch of kosher salt and a grind of black pepper. If using balsamic glaze, drizzle ~1/2 tsp over the tomato slices on each sandwich.

Close and press

Top each assembled side with the remaining bread slice, buttered side facing out. Gently press the sandwich together to distribute the filling evenly.

Toast

Heat a skillet or griddle over medium heat. Place the sandwiches in the pan and cook 3–4 minutes per side, pressing occasionally with a spatula, until the bread is golden brown and crisp and the mozzarella is melted. Reduce heat slightly if bread browns too quickly before cheese melts. Alternatively, use a panini press for 3–5 minutes until golden and cheese is melted.

Rest and serve

Remove sandwiches from the pan and let rest 1 minute, then cut each sandwich in half on a diagonal. Serve immediately.