Preheat oven to 375° F / 190° C. Line a half sheet rimmed baking tray with a silicone mat.

In a large bowl, whisk together all purpose 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 smooth and creamy, about 1 minute. Mix in the confectioner’s sugar, followed by the egg, lemon zest, and gel food coloring. Continue mixing on high speed until well combined.

Reduce mixer to low speed. Add the dry ingredients, followed by the whole milk, mixing until just barely combined.

With a rubber spatula, fold in the poppy seeds.

Scoop batter into 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. You should have about 24 rounds total. Pat the top of each round with your fingertips to smooth.

Refrigerate tray 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, about 1 minute.

Gradually add the confectioner’s sugar, alternating with the cream and mixing well after each addition. Mixture will seem dry and thick.

Add the lemon zest, lemon juice, and fine sea salt and mix until smooth, about 3-5 minutes. If necessary, add more confectioner’s sugar 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 match closely in size.

Pipe a dollop 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 3 days.