For the petal cupcakes: Preheat the oven to 350 degrees F. Line a 12-cup muffin pan with paper liners.
Whisk together the flour, cocoa, baking soda, cornstarch and salt in a large mixing bowl. Pour onto a piece of wax paper for easy pouring later.
In the bowl of a stand mixer fitted with a whisk attachment (or a large bowl using a handheld mixer), combine the sugar and egg. Whip on medium speed until the mixture is fluffy and pale yellow, 2 to 3 minutes. Add the buttermilk and continue to mix on medium speed. Add a third of the dry ingredients followed by the vanilla, mixing until combined. Add another third of the dry ingredients and then the vegetable oil, mixing between each addition and scraping down the sides and bottom of the bowl with a rubber spatula occasionally to ensure even mixing. Add the remaining dry ingredients, mixing until combined. Mix in the coffee until fully incorporated.
Divide the batter among the prepared muffin cups, filling them about three-quarters full. Bake the cupcakes until the tops spring back when lightly touched and a toothpick inserted into the center comes out clean, 18 to 20 minutes. Let the cupcakes cool completely before frosting.
For the pink buttercream frosting: In a large bowl, cream the butter until smooth with a handheld electric mixer. Beat in the confectioners' sugar a little at a time until smooth and creamy. Add the vanilla and food coloring and mix until light pink throughout.
For the pink petals: Place a saucepan filled with 1 cup of water over medium-low heat. Pour the white chocolate chips into a small, heat-safe bowl that will fit over the saucepan without touching the water. Set the bowl over the saucepan to make a double boiler, and stir the chips until they melt. Add 1 drop of the food coloring and continue to stir. After 30 seconds, add the next drop. Once the desired pink color is achieved, turn off the heat.
Put a piece of aluminum foil (about 1 square foot) on your countertop and shape into a concave curve. Place a piece of plastic wrap on top of the foil. Using a tablespoon, scoop one spoonful of the melted chocolate onto the plastic wrap and spread into the shape of a rainbow using the back of the spoon. Repeat with the remaining chocolate. Let the chocolate petals set for 10 minutes. Gently peel the petals off the plastic wrap and set aside.
To finish the cupcakes: Frost a cupcake with a large dollop of the pink buttercream frosting and gently arrange pink chocolate petals to form a flower. Repeat with the remaining cupcakes, frosting and petals.