Preheat oven to 375° F / 190° C. Line one large tray, or two smaller baking trays, with parchment or silicone mats. Set aside.

In a large bowl, whisk together flour, baking powder, baking soda, and fine sea salt.

In a separate mixing bowl or the bowl of a stand mixer, whip butter on high speed until light and fluffy.

Mix in the sugar, followed by the egg, lemon zest, and lemon oil. Continue mixing on high speed until well combined.

With a rubber spatula, fold the dry ingredients into the batter, followed by the whole milk.

Mix on low speed until just barely combined.

Transfer batter to a piping bag fitted with 1/2-inch round tip.

Pipe 1 1/2-inch rounds onto prepared trays, allowing space for the cakes to spread as they bake. Refrigerate trays for 20 minutes.

Bake, one tray at a time, in preheated oven for 9-10 minutes, or until edges are very lightly golden brown. Cool on trays for 10 minutes, then transfer to a wire rack to continue cooling while you prepare the buttercream.

Lemon Buttercream:

In a large mixing bowl or the bowl of a stand mixer, whip butter on high speed until light and fluffy.

Gradually add the confectioner’s sugar, alternating with the cream and mixing well after each addition.

Add the lemon zest, lemon juice, lemon oil, fine sea salt, and yellow gel food coloring (if using) and mix until smooth, about 3-5 minutes. If necessary, add more confectioner’s sugar or cream to adjust the consistency. The buttercream should be thick and creamy, but soft enough to pipe. Transfer to a piping bag fitted with 1/2-inch round tip.

Assembly:

Divide the cakes into pairs, choosing tops and bottoms that are as close as possible in size.

Pipe a generous amount of buttercream on the flat side of the ‘bottom’ cakes.

Top each with the second cake to create a sandwich.

Serve immediately. Store whoopie pies in an airtight container at room temperature for up to three days.