1 bent metal straw (or a plastic straw and a wooden skewer) and 1 small paper treat bag
Make the cake: Preheat the oven to 350 degrees F. Coat two 9-inch round cake pans with cooking spray and line the bottoms with parchment paper. Coat with more cooking spray and dust with flour, shaking out the excess.
Beat the butter and granulated sugar in a large bowl with a mixer on medium-high speed until fluffy, 2 to 4 minutes. Beat in the eggs one at a time, beating well after each addition.
Whisk the flour, baking powder and salt in a medium bowl. In a small bowl or liquid measuring cup, combine the milk and vanilla. With the mixer on low speed, beat in the flour mixture in three batches, alternating with the milk mixture in two batches. Beat until just combined, scraping down the bowl with a rubber spatula as needed. Fold in the sprinkles.
Divide the batter between the prepared pans; smooth the tops. Bake until the cakes are light golden brown and a toothpick inserted into the centers comes out clean, 35 to 40 minutes. Let cool 5 minutes in the pans, then turn out onto a rack to cool completely.
Meanwhile, make the frosting: Combine the egg whites, 1/3 cup granulated sugar, the cream of tartar and salt in a large bowl and beat with a mixer on medium-high speed until soft peaks form, about 4 minutes. In a medium pot fitted with a candy thermometer, heat the remaining 1 cup granulated sugar and the water over medium-high heat, undisturbed, until the thermometer registers 235 degrees F. With the mixer on medium speed, slowly drizzle the hot syrup into the egg white mixture. Increase the mixer speed to high and continue to beat until the frosting is no longer steaming, about 3 minutes. Add the butter, one piece at a time, and continue to beat until thick and smooth, about 2 more minutes. (It's OK if the frosting looks curdled -- keep beating and it will eventually smooth out.) Add the vanilla and beat until incorporated.
Assemble the cake: Place 1 cake layer on a cake stand and spread with 1 cup of the frosting. Place the remaining cake layer on top. Frost the top and sides of the cake with a thin layer of frosting (this is the crumb coat). Refrigerate until the frosting is set, about 30 minutes.
Meanwhile, roll out the fondant on a confectioners' sugar-dusted surface into a long rectangle, about 2 by 7 3/4 inches. Wrap the fondant around the bent metal straw, leaving about 2 inches bare at the bottom. Pinch the seam to close, then roll the straw and fondant between your hands to smooth it out. (If using a plastic straw, insert a wooden skewer through it for stability.)
Cover the cake with the remaining frosting in a smooth, even layer. Mark a spot on the top of the cake that's slightly off center (this is where the straw will go). Sprinkle confetti sprinkles on the cake so they radiate out from the marked spot and go down the side of the cake.
Rub or brush a thin layer of corn syrup all over the fondant-covered straw, then roll in confetti sprinkles, pressing to adhere. Insert the straw into the cake at the marked spot, then place a small treat bag on the bent end, using a little corn syrup to stick it on, if necessary. Refrigerate until ready to serve.