Whisk together the sugar, flour, cocoa powder, baking soda, baking powder and salt in a large bowl. Add the buttermilk, vegetable oil and warm water and whisk to combine. Then add the egg and vanilla extract and whisk until the batter is smooth. Line a cupcake pan with cupcake liners and fill each cup 2/3 of the way full with batter. Bake at 350F for 20-25 minutes. Cool in the pan for 5 minutes, then transfer to a wire rack and cool completely.
While the cupcakes are cooling, make the buttercream. Place the butter in a bowl and whip with an electric mixer until pale and fluffy. Add the vanilla extract and mix to combine. Add the confectioner's sugar one cup at a time, mixing well with each addition. Then whip for 3-5 minutes, until fluffy. Dye the buttercream a golden yellow color.
To decorate, spread a thin layer of buttercream onto the surface of each cupcake. Place the remaining buttercream in a piping bag fitted with a star-shaped piping tip. Pipe rings of petals on the cupcake, starting from the outside and working towards the center. 5. Fill the center with mini chocolate chips and enjoy!