Cakes:

Preheat oven to 400° F / 205° C. Line two baking trays with silicone mats, and set aside until ready to use.

In a large bowl, combine the flour, baking soda, baking powder, and salt. Set aside.

In a separate mixing bowl or the bowl of a stand mixer, whip butter, sugar, and vanilla bean paste on high speed until light and fluffy. Add the egg and mix until well combined.

In 3-4 additions, add the dry ingredient mixture to the batter, followed by the milk. Mix until well combined.

Add the light green food coloring, a few drops at a time until the desired shade is achieved. (Be careful not to go too dark–you want the cakes to be light enough to create a contrast with the dark green lines you’ll be painting on top!)

Transfer batter to a piping bag. Pipe 1.25-inch / 3-centimeter rounds on prepared trays, allowing a good amount of space in between. With damp fingertips, pat down the tops of each cake to create a smooth, evenly domed surface.

Ina small bowl, mix a few drops of dark green gel food coloring** with a drop or two of water. (The consistency should be smooth, but still somewhat thick. If the mixture is too watery, it will bleed when applied to the cakes.) Using a thin paintbrush, paint “watermelon” lines on the top of each cake.

Bake in preheated oven for 8-10 minutes, or until cakes are just beginning to brown around the edges. Do not overbake.

Transfer to a wire rack and allow to cool completely before filling.

While cakes are baking and cooling, prepare the watermelon buttercream.

Buttercream:

In a large mixing bowl or the bowl of a stand mixer, whip butter until light and fluffy. Gradually add the icing sugar, mixing well after each addition. Mix in the milk/whipping cream and a pinch of salt, then whip until smooth and creamy, about 3-5 minutes. If necessary, add a bit more cream or icing sugar to adjust the consistency.

Add the pink food coloring and the watermelon flavoring, adjusting as necessary until desired color and flavor are achieved.

Using a wooden spoon or rubber spatula, gently fold chocolate chips into the buttercream.

When cakes have cooled, transfer watermelon buttercream to a piping bag. Pipe a generous amount of buttercream on the flat side of one cake, then top with another cake to create a sandwich.

Store watermelon whoopie pies in an airtight container at room temperature for up to 3 days, or in the refrigerator for up to one week. Be sure to add sheets of parchment or wax paper between layers to prevent the cakes from sticking together. Alternately, you can wrap each individual pie in clingfilm/plastic wrap for freshness and convenience.