Preheat the oven to 200°F. Line two baking sheets with parchment paper. Using an electric mixer, beat the egg whites in a large bowl until foamy. Add the lemon juice and continue to beat until soft peaks have formed. Gradually add in the sugar and beat until it is all dissolved and stiff peaks form. Lastly, beat in the vanilla until just incorporated (you don’t want to over mix). Use a rubber spatula to gently fold in the pistachios. Using two spoons, line the baking sheets with small dollops of meringue. Leave about an inch between each meringue. Bake for an hour and a half until the meringues are dried out and crisp. (You may have to adjust the time, as every oven will cook the meringues differently.) Let cool.
In a small saucepan, combine the chocolate, butter and salt over low heat. Melt the mixture until smooth and creamy. Pour the mixture into a glass or cup deep enough to dunk the meringues into it. When the meringues are cool, dip each one half way into the chocolate mixture. Lay them back onto the baking sheet and sprinkle the chocolate half with pistachios when the chocolate is still wet. Let dry and serve!
When beating egg whites, “soft peaks” is the stage where the whites are just beginning to take shape. If you flip your electric mixer upside down, the peaks will just slightly hold. “Stiff peaks” occurs when you continue to beat and the whites completely hold their shape. Flipping the mixer will result in firm peaks standing upright (just bending a little at the top). It is easy to overbeat the whites, so if you notice a grainy, dull texture, you have gone too far.