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1
Preheat the oven to 350 F. Line 2 large cookie sheets with parchment paper.
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2
Stir together the flour, baking powder and salt in a medium sized bowl. Set aside.
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3
In a large bowl, beat the butter, sugar and lemon zest until pale and fluffy. Beat in the beaten eggs and vanilla. Beat in half the flour mixture, then mix in the milk, and then mix in the remaining flour mixture.
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4
Drop equal sized scoops of cookie dough onto the parchment-lined cookie sheet, spacing them two inches apart. Flatten each cookie with a spatula. Try to make them all the same size/shape.
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5
Bake for 8-10 minutes until very light brown. Do not over-bake these. Remove pan from oven and set it on a rack. Allow cookies to cool for 5 minutes, then transfer cookies to a cooling rack and let them cool completely. Repeat until cookies are all baked.
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6
This recipe makes approximately 5-6 dozen single cookies, which stack together to make around 2 1/2 dozen whoopie pies.
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7
While cookies are cooling, make the lemon buttercream filling. Place the butter, shortening, lemon juice, and vanilla into a large bowl. Beat until pale and creamy. Gradually beat in the icing sugar and continue beating for 2 minutes. Add the hot water if the filling seems too thick to spread or pipe.
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8
Pipe the filling onto the flat underside of half of the cookies. This amount of filling is usually enough but not too much, just the way we like them. Place the remaining cookies on top to complete the whoopie pies.
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9
Serve or freeze almost immediately because at room temperature the buttercream filling tends to melt.
