Preheat oven to 325°F. Sift together flour, cornstarch, baking powder, and lemon zest in a medium bowl and set aside.
In a large bowl, beat butter, shortening, and sugar on medium-high speed for 3-4 minutes until light and fluffy.
Add room temperature egg, vanilla, and almond essence; beat on medium speed for 1-2 minutes until smooth.
Add dry ingredients in 2-3 additions on low speed, mixing just until combined and dough is soft but not sticky.
Roll dough on a lightly floured surface to 1/4-inch thickness. Cut into bunny shapes and place on parchment-lined baking sheets.
Bake for 9-10 minutes until edges set but cookies remain pale. Cool on baking sheet for 5 minutes, then transfer to a wire rack to cool completely.
For frosting, beat softened butter on medium speed until creamy. Gradually add sifted powdered sugar, mixing on low speed.
Add vanilla essence and milk one tablespoon at a time, beating between each addition until smooth and spreadable.
Transfer frosting to piping bags fitted with small round or star tips.
Pipe frosting onto cooled cookies to create bunny features and decorate with sugar sprinkles and black sugar pearls.
Let frosting set at room temperature for 30 minutes before serving or storing.