In a mixer with paddle attachment cream the butter and the sugar.

Add the egg whites gradually and then mix in the vanilla and lemon extracts.

Add the flour and mix until just well mixed.

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.

While waiting for the cookies to cool, 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.