Preheat the oven to 300 degrees.
Whisk together the flour, salt and baking powder.
Cream together the butter and sugar with the lemon zest, vanilla, and vanilla bean seeds. Add the egg and mix until completely incorporated.
Add the cream and the dry ingredients to the butter mixture, alternating with each addition, and mix just until a homogenous dough is formed.
Form a flat disc with the dough and wrap it in plastic wrap. Refrigerate overnight.*
Roll out the dough to approximately 1/8-inch thickness and cut into desired shapes (if baking for decorations to trim a tree, it is important to remember to punch a small hole out now, before baking the cookies.
Bake the cookies for about 10 minutes, or until the edges just turn golden.
Cool completely before decorating.
*Note: You can skip this step if you want to bake the cookies right away, but refrigerating overnight accomplishes two things. 1. It chills the dough, making it easier to roll and cut, and 2. It allows the flour to absorb moisture from both the cream and the egg, making for a much nicer dough and a better cookie).