In a medium, heavy-bottomed sauce pan, whisk together 2/3 cup granulated sugar, 1/4 cup cornstarch, and 1/4 teaspoon kosher salt. Add 2 cups milk and 2  eggs, whisking until combined. The mixture will be a little lumpy, but that's okay, I promise! Bring to a boil over medium heat, whisking constantly until the mixture is thick and creamy, 5 to 8 minutes.

Combine 1 cup water, 1/2 cup cubed unsalted butter, and 3/4 teaspoon kosher salt in a medium, heavy-bottomed saucepan. Cook over medium-low heat, stirring frequently, to melt the butter. Once the butter has melted, increase the heat to medium-high and bring to a simmer. Remove the pan from heat and immediately stir in 1 1/4 cups all-purpose flour and 2 tablespoons plus 2 teaspoons granulated sugar with a wooden spoon. Continue to stir for about 2 minutes (or until the mixture has a paste-like consistency), then place over medium heat and stir rapidly for 1 to 2 minutes, or until the dough pulls away from the sides of the pan and the bottom of the pan is clean. The dough should be glossy and smooth, but not dry.

Scape the dough into a pastry bag fitted with an open star tip. Refrigerate until cool, about 1 hour. While the choux pastry is chilling, prepare your oven. Position a the racks in the upper and lower thirds of the oven and preheat to 375 (F). Line two sheet pans with parchment paper and set aside.

Once the dough is chilled, it's time to pipe the eclairs. Starting at the side of the parchment farthest away from you, hold the tip of the pastry bag 3/4 inch above the parchment and apply gentle, steady pressure as you pipe the first eclair. When the eclair is about 6 inches long, begin to lessen the pressure, and then stop it as you brng the dough back over itself, leaving a 1/2 inch curl at the end of the eclair. Pipe 5 more eclairs on the parchment, leaving about 2 inches between each eclair. Repeat with the second sheet pan, piping 6 more eclairs.

Wet the tip of your finger and press down the tip of each eclair, then spray them lightly with water. Place the sheet pans in the oven, placing one on the upper third and the second on the lower third, and immediately lower the oven temperature to 350 (F). Bake for 20 minutes, then switch the pans' positions, and bake for an additional 20 minutes. After 20 minutes, lower the oven to 325 (F) and bake for an additional 20 minutes, or until golden brown. Lower the temperature to 300 (F) one last time and bake for 10 minutes longer, or until the puffs are light and hollow (tap the bottom of an eclair to test this).

In the top pan of a double boiler (or, make a homemade version by placing a heatproof bowl on top of a medium saucepan with an inch or two of simmering water—make sure the water doesn't touch the bottom of the bowl!), combine 2 cups confectioners' sugar, 4 ounces coarsely chopped dark chocolate, 5 tablespoons unsalted butter, 1/4 cup water, 1 1/2 teaspoons vanilla extract, and 1/4 teaspoon kosher salt. Cook over medium heat, stirring occasionally, until the chocolate and butter are completely melted. Remove from heat and stir until smooth; use immediately.

First, set up your work station: transfer the pastry cream into a piping bag with the tip of your choice (I re-used the open star tip), and pour the chocolate glaze into a shallow bowl.

Use a serrated knife to slice each eclair in half lengthwise. Use the piping bag to fill the bottom half of each eclair with 3 to 4 generous dollops of pasty cream.

Next, glaze the eclairs. Hold the top half of an eclair parallel to the surface of the glaze and dip it into the glaze to coat the top. Pull it out of the glaze, still parallel to it, and turn the eclair upward, letting the excess glaze run off, then rotate slight to one side to stop the glaze from dripping. Repeat with the remaining eclair tops, placing each top back on its bottom half as you go. Serve immediately.