Cookies:

Preheat oven to 350° F / 176° C. Line a large baking tray, or 2 smaller trays, with a silicone mat.

In a medium bowl, whisk together all purpose flour, cornflour, baking powder, and kosher salt.

In a separate bowl or the bowl of a stand mixer, mix butter and granulated sugar on high speed until soft and fluffy.

Mix in the egg and vanilla extract. Mixture will look slightly separated.

Add the dry ingredients, alternating with the buttermilk and mixing on low speed until a soft batter forms.

Scoop batter into a disposable piping bag fitted with 1/2-inch round tip. Pipe onto prepared tray in 2-inch rounds, allowing 2-3 inches of space in between each round. (Depending on the size of they tray(s) you’ve chosen, you might need to bake in multiple batches. Do not overcrowd the tray—these will spread as they bake!)

Bake in preheated oven, one tray at a time, for 11-14 minutes, or until cookies spring back lightly when touched.

Cool on tray for 5 minutes then transfer to a wire rack to cool completely. Repeat with remaining batter.

While cookies cool, prepare the icing.

Icing:

Line a large tray with parchment or wax paper. Place wire rack over top.

In a small bowl, whisk together confectioner’s sugar, water, corn syrup, lemon juice, and vanilla extract.

Scoop half of the icing into a second small bowl, and whisk in the cocoa powder. Add water, 1 teaspoon at a time, to adjust consistency.*

Flip cookies over. (You’ll ice the flat sides, not the domed sides.) With a small stepped palette knife, ice half of the top of each cookie with vanilla icing, then return to wire rack. (Icing will drip.) Allow vanilla icing to set, about 15 minutes.

Repeat with the chocolate icing.

Allow cookies to rest uncovered until icing sets, about 15 minutes.

Store black and white cookies in an airtight container at room temperature for up to 3 days.