Special equipment: 2 pastry bags, an open tip and 1/4-inch round tip
For the croquembouche: Place a rack in the middle of the oven and preheat to 400 degrees F. Line a baking sheet with parchment paper.
In a medium saucepan, combine the butter, sugar, salt and 1 cup water. Bring to a boil over medium heat. Remove from the heat and dump the flour directly into the pan. Stir the mixture into a paste with a wooden spoon. Return the pan to medium heat. Stir the mixture until it pulls away from the sides of the pan and is slightly shiny, about 3 minutes.
Transfer the paste to a medium bowl and beat with a wooden spoon until slightly cooled, 2 to 3 minutes. Add the eggs one at a time, incorporating each one thoroughly before adding the next. Before adding the last egg, beat it in a small bowl. Add it by tablespoons to the mixture, just until the batter is smooth and tight. If the batter starts to get loose, do not add all of the last egg.
Fill a large pastry bag fitted with an open tip with the batter. Pipe twelve 1-inch circular mounds about 2 inches apart onto the baking sheet. (Alternatively, use a large spoon to form the dough into mounds.) Dab the tops of each puff with a fingertip dipped in water to smooth.
Bake in the middle of the oven for 20 minutes. Remove the puffs from the oven and use a sharp paring knife to pierce each side. Return the puffs to the oven until dry and crisp, another 5 minutes. Let cool on a rack. Repeat with the remaining batter.
For the meringue frosting: Bring a few inches of water to a boil in a saucepan that can hold a mixing bowl above the water. Whisk the sugar, lemon zest and juice, egg whites, cream of tarter and salt by hand in the mixing bowl. Set the bowl above the boiling water and continue whisking until the mixture is hot to the touch and the sugar dissolves, 1 to 2 minutes. Transfer the mixture to a stand mixer fitted with the whisk attachment and beat at medium-high speed until the meringue is cool and holds a soft peak, about 5 minutes. Add a few drops of desired food coloring if using and continue beating until it holds stiff peaks.
To assemble: Fit a pastry bag with a 1/4-inch round tip (a long eclair filling tip works great for this) and add the frosting to the bag. Insert the tip into the slit in the side of each puff and fill with frosting.
Melt the white chocolate in a bowl over a pot of simmering water or in a microwave. Start by dipping a small bit of each puff into the melted white chocolate. Create an even round layer with the puffs on a 12- to 14-inch serving platter, using the chocolate to adhere. Place the platter in the refrigerator and allow to set for 10 minutes. Repeat with additional layer of puffs on top of the first, decreasing in size to create a "tower" effect and reheating the white chocolate to melt as needed. In between each layer, place the plate back in the fridge and allow to set for 10 minutes. When you reach the top, use the remaining white chocolate to drizzle over the top of the tower. Return the platter to the fridge and remove 20 minutes before you are ready to serve.
Consumption of raw or undercooked eggs, shellfish and meat may increase the risk of foodborne illness.