Preheat your oven to 175° C / 347° F and line two baking sheets with baking paper.
In a bowl combine butter and sugar and whisk for at least 5 minutes or until it's creamy and fluffy. Add one egg at a time until all three are incorporated.
Add the lemon juice
Mix the flour, baking powder and lemon zest and gently fold into the dough.
Fill the dough into a piping bag with a big round tip and pipe 14 circles with a diameter of ca. 5cm on each baking sheet. Bake for 15 min and let them cool on a cooling rack.
Meanwhile in a bowl combine the mascarpone, the creme fraîche, lemon juice and the confectioners' sugar and whisk until well combined.
Fill into a piping bag with a big round tip and pipe a generous border around the flat side of half the baked circles.
Add a tsp of jam in the middle and assemble the whoopie pies.