Cream the butter with a paddle attachment then mix in the sugar. Add the egg whites gradually and then mix in the vanilla and lemon extracts. Add the flour and mix until just incorporated. With a small (1/4-inch) plain tip, pipe 1-inch sections of batter onto a parchment-lined sheet pan, spacing them 2 inches apart as they spread. Bake in a preheated 350 degree oven for 10 minutes or until light golden brown around the edges. Let cool on the pan.
Cookie Filling: In a small saucepan over medium flame, scald cream. Pour hot cream over chocolate in a bowl. Whisk to melt chocolate, add zest and blend well. Set aside to cool (the mixture will thicken as it cools). Spread a thin amount of the filling onto the flat side of a cookie while the filling is still soft and press the flat side of a second cookie on top. Repeat with the remainder of the cookies.