• Preheat the oven at 350 F / 180 C and line your baking tray with parchment paper.
• Add in the butter and sugar in your stand mixer (you can also use a hand mixer for this), and mix it on medium-high speed till it's light and creamy in texture.
• Add in the egg, lemon juice, vanilla extract, and zest and mix till the egg is fully incorporated into the mixture.
• Now add the salt and cornstarch into the flour and mix well. Add this to the wet mixture in batches and mix at low speed till the dough comes together.
• Roll the dough between two sheets of parchment/wax paper to a 1/4th inch thickness and cut out egg-shaped cookies and bake them for 18 - 20 min or until cookies turn light golden.
• Let the cookie cool down completely on a wire rack before decorating.

For Royal icing:

• Beat the egg white at low speed till it starts to froth. Now increase the mixer speed to high and slowly start adding in the sugar. After you have added all the sugar, the mixture should be very thick and white.
• (You can use it as is or reduce the consistency by adding 1-2 tsp of water before adding the color of your choice. The general rule of thumb is to use the thicker one for outline and use a little thinner version for filling the entire cookie.)
• Divide the icing into two bowls and add yellow or any other food color of your choice to one. Transfer half of both (white and yellow) colors to separate piping bags. Thin the remaining icing with 1-2 tsp of water.
• Outline the cookies with thick icing and pour the thinner icing over the cookies; let it dry completely. Decorate with thick icing in the piping bags.