1In a large bowl, beat the butter, sugar and salt with an electric mixer for 2 minutes, until pale and fluffy. Beat in the egg yolk and vanilla.
2Add the flour and grated Tonka bean and stir just until you have a soft dough. Shape it into two logs, wrap and refrigerate for an hour (or up to a few days) or freeze for up to 6 months.
3When ready to bake, slice 1/4" thick and bake on a parchment-lined sheet at 350F for 15 minutes, or until pale golden around the edges.
4 Transfer to a wire rack to cool.