Special equipment: a 12-cup Bundt pan; a 2-inch "D"-shaped cookie cutter
For the cake: Preheat the oven to 350 degrees F. Line a half-sheet pan with parchment and spray with nonstick spray. Spray a 12-cup Bundt pan.
In the bowl of a stand mixer fitted with the paddle attachment, cream the butter and sugar until light and fluffy, 5 to 10 minutes, stopping a few times to scrape the sides and bottom of the bowl. Add the eggs one at a time, beating for 30 seconds after each addition. Beat in the vanilla.
In a large bowl, whisk the flour, salt and baking powder together for 30 seconds to distribute the leavening. With the mixer on low, alternate additions of the flour mixture and the buttermilk. Using a large rubber spatula, scrape the bottom and sides of the bowl to ensure the batter is well mixed.
Transfer a generous one-quarter to one-third of the batter to a mixing bowl. Add the green food coloring and stir until the batter is a uniform color. Spread the green batter in the prepared sheet pan and bake until just baked through and firm but not yet springy, 10 to 15 minutes. Let cool, then freeze for 20 to 30 minutes until firm.
Put one-third of the undyed batter in the bottom of the prepared Bundt pan. Using a 2-inch "D"-shaped cookie cutter (you can use other shapes as well) cut 14 or 15 letters from the green cake. Stand the letters, upside-down, in the Bundt pan, with the straight edges toward the center of the pan. Don't press the letters all the way into the batter, just enough to keep them upright. Arrange the letters as close together as you can while still keeping them properly angled.
Fill a large pastry bag with the remaining undyed batter and pipe it into the Bundt pan, making sure to fill any hard-to-reach areas and covering the letters completely (see Cook's Note). Bake until the cake is golden brown and springs back when gently touched, 50 to 60 minutes. Allow to cool completely in the pan.
For the Swiss meringue frosting: In a stand mixer bowl, combine the egg whites, sugar and salt. Place over a medium saucepan of simmering water to create a double boiler, making sure the water doesn't touch the bottom of the bowl (you want the steam to heat the egg whites, not the simmering water). Whisking constantly, heat the egg white mixture until the sugar has completely dissolved and feels hot to the touch. (If you are nervous about uncooked eggs, attach a thermometer the bowl and make sure the mixture reaches 170 degrees F.) Transfer the bowl to the stand mixer and beat the mixture on high with the whisk attachment until very stiff, shiny white peaks form and the meringue is room temperature, about 10 minutes.
To assemble the cake: Run a long, thin knife around the edge of the Bundt pan to release the cake. Place a cake platter, face down, on top of the Bundt and flip the pan over. Gently lift away the pan, leaving the cake on the platter.
Coat the cake with an even layer of Swiss meringue. Serve immediately or store in the refrigerator.
For shapes or letters with an open center, it's helpful to pipe a bit of batter into the open area before placing in the Bundt pan