Pour milk into a small, heavy-bottom saucepan and simmer over medium heat until just before boiling; you'll know it's done when it's steaming. Remove from heat.
In a heatproof bowl, whisk together the eggs, salt, sugar, and flour until combined (I used my pyrex 1qt cup I measured the milk in).
Using a ladle, scoop some of the hot milk and slowly pour it into the egg mixture while whisking constantly. It's very important you do this slowly as you want to temper the eggs, not cook them. Do this until about half of the milk is in the egg mixture.
Bring the saucepan back to the stove over low heat. Whisk the hot milk while pouring the egg mixture back in and continue to whisk constantly until thickened. This will take 3-4 minutes; don't stop whisking!
Remove from heat, stir in vanilla, and place in a small bowl. Cover with plastic wrap, allowing the wrap to touch the top of the pastry cream (this will prevent the cream from forming a skin).
Place in the fridge to chill for at least thirty minutes (this can be made a couple days in advance).
Lightly dust your counter or cut-proof board with flour. Roll out puff pastry and cut each sheet into thirds.*
Using a fork, prick each sheet entirely (see image earlier in the post for example).
Place parchment paper on your baking sheet(s). Add puff pastry and don't allow them to touch.
Place in the oven and bake for about 15 minutes, until the bottoms are golden. Remove from oven and let cool about fifteen minutes.
While the puff pastry is cooling, make the icing by adding powdered sugar and milk into a bowl and whisk together until thoroughly combined; the icing will be thick, but pour-able.
Melt the chocolate in a small double boiler or in the microwave using a small bowl.
Take one sheet and top with a quarter of the pastry cream. Spread all over evenly. Top with another sheet and repeat with another quarter of the cream.
Add the third sheet upside down and top with half of the icing. You can do this by adding a couple tablespoons at a time or streaming it down the pastry. Using a butter knife or icing spatula, spread evenly.
Using a small spoon, drizzle four or five lines of chocolate down the pastry, lengthwise.
Take your butter knife and create a feathered look by dragging the knife crosswise through the chocolate lines, about an inch apart. Then drag the knife through lines in the opposite direction.
Repeat steps 15-18 for the remaining puff pastry sheets.
Chill in the refrigerator for at least three hours before cutting.
Use a serrated knife to carefully cut into pastry. Each pastry log can be cut into 5-6 servings, depending on preference.
Depending on the brand of puff pastry, the measurements of the sheets may vary. I used Trader Joe's brand, which equaled to 10x10-inch pastry sheets. If the puff pastry you purchased don't have even dimensions, cut them in thirds lengthwise.