Preheat your oven to 400°F. Line two large baking sheets with parchment paper. This takes 10 minutes total.

Remove the two puff pastry sheets from the freezer and place them on the counter. Let them thaw at room temperature for exactly 20 minutes until they're soft and foldable but still very cold to the touch. Checkpoint: You are ready when the pastry bends without cracking.

While the pastry thaws, prepare the chocolate filling. Chop 8 ounces of semi-sweet chocolate and 4 ounces of dark chocolate into small, uniform pieces (about the size of peas).

Add 3 tablespoons of butter to a small saucepan and place over low heat. Medium-low heat is important—if too hot, the chocolate will seize and become grainy (about 30 seconds for the butter to melt).

Once the butter is melted and foamy, immediately add the chopped chocolate. Stir constantly with a silicone spatula for 2-3 minutes until completely smooth and glossy. The mixture should look like liquid silk with no visible chocolate chunks.

Remove the pan from heat. Add 2 tablespoons of heavy cream and 1 teaspoon of vanilla extract, then stir for 30 seconds until fully combined. Add a pinch of sea salt and mix once more. The filling should be thick enough to hold its shape but spreadable. Let it cool for 5 minutes on the counter—this prevents it from melting the cold pastry.

Carefully unfold the first thawed puff pastry sheet on a clean, dry work surface. Do not tear it. Gently cut it into 4 equal rectangles using a sharp knife in 2-3 smooth strokes (don't saw back and forth, which crushes the layers). You now have 4 rectangles. Repeat with the second sheet for 8 total rectangles. Place 4 rectangles on the prepared baking sheets, spacing them 2 inches apart.

Spoon exactly 2 tablespoons of the cooled chocolate filling onto the center of each of the 4 pastry rectangles on the baking sheets. Use the back of the spoon to spread it gently into a thin, even layer, leaving a clean half-inch border all around the edges—this border must stay clear so the edges can puff and seal properly.

Take the remaining 4 pastry rectangles and place one directly on top of each chocolate-filled rectangle, pressing down very gently with your fingertips. Do not press hard or you'll crush all the air layers.

Using the tines of a fork, gently press around the entire border (about half an inch from the edge) to seal the two pastry sheets together and create a decorative pattern. This also ensures they don't separate during baking. Be gentle—you're crimping, not mashing.

In a small bowl, crack 1 large egg and whisk it with 1 tablespoon of water for exactly 30 seconds until light and frothy. This is your egg wash. Using a pastry brush, brush the top surface of each pastry generously with the egg wash. This creates the golden-brown shine and helps the pastry puff evenly. Do not brush the edges where they're sealed, or they may slip apart.

Place both baking sheets in the preheated 400°F oven on the middle rack. Set a timer for exactly 15 minutes. At 12 minutes, peek through the oven window—the pastries should be starting to puff and turn light golden.

Bake for a total of 15-18 minutes. The pastries are done when they are deep golden brown on top (like a caramel color, not pale), puffed up noticeably, and feel slightly firm to a gentle touch. The bottom should be crispy when you lift one slightly with a spatula.

Remove both baking sheets from the oven. Let the pastries cool on the baking sheets for exactly 5 minutes—this allows the interior to set without cooling too much. Checkpoint: You are ready when the pastries are still slightly warm but hold their shape firmly.

Using a wide spatula, carefully transfer each pastry to a wire cooling rack. Let them cool for another 10-15 minutes until they are warm or room temperature (they will continue to firm up).

Once cooled, place 2 tablespoons of powdered sugar in a fine-mesh sieve. Hold the sieve about 6 inches above the pastries and tap gently to dust each one evenly with a light snow of powdered sugar.

Rinse out the sieve (or use a clean one) and add 1 tablespoon of cocoa powder. Tap gently to dust each pastry with cocoa powder on top of the powdered sugar. This creates a beautiful two-tone finish.

Serve immediately or within 1 hour while the pastry is still crispy. If stored longer, the pastry will soften but will still taste delicious.