In a stand mixer with a paddle attachment or in a bowl with an electric mixer, beat together butter, icing sugar vanilla and salt until light and fluffy, about 2-3 minutes. Stir in flour just until combined.
Press dough into a 9-inch round tart pan with a removable bottom (or alternately, a 9-inch round cake pan, lining the bottom with a round of parchment paper.) Flatten dough with the palm of your hand to make a smooth surface. Using a sharp knife, score the surface of the dough in to 16 wedges. Prick each wedge with a fork twice. Refrigerate pan with dough until firm, about 30 minutes.
Meanwhile, preheat oven to 300F.
Bake cookies in preheated oven until firm and pale golden, about 50-55 minutes. Remove from oven and let cool for 5 minutes, then, using a sharp knife, cut through the score lines you made before baking, wiping off the blade between each cut. Allow to cool completely in the pan.
Meanwhile, prepare glaze by whisking together the icing sugar and lemon juice until smooth. Tip: Be sure to use a bowl that will allow you to dip the cookies in flat from end to end, so it should be at least 5-inches wide. That said, don't use too big a bowl or your glaze will be too thin a layer and you may bottom-out when you dip).
Remove cookies from pan to a rack. Holding a cookie by the sides, dip the top of the cookie into the glaze to coat. Hold point side down and allow the excess to drip off. Gently remove any drip at the point end by running it along the edge of the bowl. Place on to rack. Once you've done 4 or 5, sprinkle those ones with some of the chopped pistachio. Repeat until you have glazed and sprinkled all the cookies. Allow to stand until the glaze is set, about 1 hour. Store in a flat even layer in an airtight container. These keep well stored at room temperature for about a week. These would also freeze well!