Position a rack in the lower third of an oven and preheat to 325°F Grease and flour a beehive cake pan; tap out excess flour.
To make the cake, over a sheet of waxed paper, sift together the flour, baking powder and salt; set aside.
In the bowl of an electric mixer fitted with the flat beater, beat the butter on medium speed until creamy and smooth, about 1 minute. Add the granulated sugar and lemon zest and continue beating until light and fluffy, about 5 minutes, stopping the mixer occasionally to scrape down the sides of the bowl. Add the eggs a little at a time, beating well after each addition. Beat in the vanilla just until incorporated, about 30 seconds.
Reduce the speed to low and add the flour mixture in three additions, alternating with the milk and beginning and ending with the flour. Beat each addition just until incorporated, stopping the mixer occasionally to scrape down the sides of the bowl. Add the lemon juice and beat for 30 seconds.
Spoon the batter into the prepared pan, spreading the batter so the sides are higher than the center. Bake until the cake begins to pull away from the sides of the pan and a toothpick inserted into the center comes out clean, 55 to 60 minutes. Transfer the pan to a wire rack and let the cake cool in the pan for 10 minutes.
Meanwhile, make the glaze: In a small saucepan over medium heat, stir together the honey, lemon juice and salt and bring just to a simmer, about 2 minutes. Remove from the heat.
Tap the cake pan gently on a work surface to loosen the cake. Set the rack over a sheet of waxed paper, invert the pan onto the rack and lift off the pan. Using a pastry brush, brush the warm cake with the glaze. Let the cake cool completely, at least 2 hours, before assembling and decorating.
To make the buttercream, in a small bowl, using a handheld mixer, beat the butter on medium speed until smooth and creamy, about 1 minute. Add the confectioners' sugar, milk, vanilla and salt and continue beating until light and fluffy, 2 to 3 minutes more.
To make the royal icing, in a small bowl, stir together the confectioners' sugar and the 2 teaspoons milk until smooth. If necessary, add more milk 1/2 teaspoons at a time until the icing is thick but still pourable.
Stand one half of the cooled cake vertically on its base. Using a serrated knife, level the flat side of the cake by trimming off 1/8 to 1/4 inch from the edge. Repeat with the other cake half.
Using an offset spatula, spread a thin layer of buttercream, about 1/2 cup, on the cut side of one of the cake halves. Place the cut side of the other cake half against the frosted side and gently press to secure the two halves; using the spatula, smooth the buttercream at the seam.
Using a large spatula, carefully transfer the cake to a serving platter. Drizzle with the royal icing, making sure to cover the frosted seams of the beehive. Decorate with sugar honeybees.