Instructions

for the lemon syrup

Bring water and sugar to boil in small saucepan. Cook, stirring constantly, until sugar has dissolved, about 1 minute. Remove saucepan from heat, stir in lemon zest, and let cool completely, about 30 minutes. Discard lemon zest. (Refrigerate for up to 1 week; bring to room temperature before using.)

for the cake

Adjust oven rack to middle position and heat oven to 350 degrees. Grease 4-inch round cake pan. Microwave butter and milk in small bowl until butter has melted, about 30 seconds; set aside to cool.
Using stand mixer fitted with whisk attachment, beat egg and half yolk, sugar, and honey on medium-low speed until combined, about 30 seconds. Increase speed to medium-high and beat until batter is slightly thickened and ivory in color, about 3 minutes, scraping down sides of bowl occasionally.
Sift flour and cornstarch through fine-mesh strainer into small bowl. Using silicone spatula, fold flour mixture into egg mixture, taking care to scrape bottom of bowl, until no streaks of flour remain (be careful not to overmix). Add melted butter mixture and vanilla and continue folding with silicone spatula until thoroughly combined.
Transfer batter to prepared pan. Using chopstick, swirl batter to release any small air pockets, then gently tap pan on counter to settle batter. Bake until cake is golden brown, center of cake is firm to touch, and toothpick inserted in center comes out clean, 20 to 22 minutes. Clean and dry mixer bowl and place it in freezer.
Carefully remove cake from oven and gently tap pan on counter to settle cake. Immediately remove cake from pan and let cool completely on wire rack, about 30 minutes.

for the frosting

Meanwhile, using stand mixer fitted with chilled bowl and whisk attachment, whip cream, sugar, and condensed milk on medium-low speed until foamy, about 1 minute. Increase speed to medium and whip until medium-stiff peaks form, about 4 minutes (be careful not to overbeat). If desired, transfer 1½ cups frosting to separate bowl and fold in drops of food coloring until desired color is achieved. Cover and refrigerate frosting(s).

to assemble the cake

Using serrated knife, trim mounded top off cake; discard (or snack on!) trimmings. Score 2 evenly spaced horizontal lines around sides of cake. Following top scored line, cut first layer of cake. Carefully remove layer. Following remaining scored line, cut remaining cake into 2 layers. Carefully remove top layer.
Lightly brush top of bottom cake layer with lemon syrup; transfer to piece of parchment or wax paper cut to fit 6-inch square takeout container. Using mini offset spatula or large spoon, transfer 2 tablespoons white frosting to center of cake layer and spread it evenly over surface. Place second cake layer on top and press gently to adhere. Repeat brushing with syrup; spread 2 tablespoons white frosting evenly over second layer. Place third cake layer on top and repeat brushing with syrup; discard remaining syrup. Frost top and sides of cake with ½ cup white frosting to create thin layer, then refrigerate cake for 15 minutes.
Frost top and sides of cake with colored frosting, if using, or remaining white frosting. If desired, use any remaining frosting to create decorative design on top. Wipe away any extra frosting on parchment square. Using edges of parchment to assist you, carefully transfer cake and parchment square to takeout box. Serve immediately or store in refrigerator for up to 24 hours.