Carefully unroll the puff pastry sheet, keeping the parchment paper underneath. Use a round cookie cutter to cut out an even number of circles—these will be the bases and tops. Place a small amount of tomato puree in the center of each base, add 1 or 2 capers, and a piece of mozzarella cheese slice. Moisten the edges with a little water, place a top circle over each base, and press the edges to seal well.

Use a smaller round cookie cutter or a glass to gently press around the filling to help seal the edges.

Brush the top of each mini pizza with egg yolk mixed with a little water. Arrange the mini pizzas on a baking sheet lined with parchment paper. Bake at 400°F (200°C) for about 15 minutes, or until the tops are golden brown.