Add the softened butter and granulated sugar to a large bowl. Beat until smooth and creamy.
Add the vanilla bean seeds or vanilla powder, vanilla extract, and fleur de sel. Mix until combined.
Add the flour and mix just until a crumbly dough forms.
Gently press the mixture together with your hands until it forms a cohesive dough.
Shape the dough into an even log approximately 2 inches (5 cm) thick.
Wrap tightly in plastic wrap and refrigerate for 1 hour, or until firm.
Preheat the oven to 350°F (175°C) and line a baking sheet with parchment paper.
Unwrap the chilled dough and roll the outside in crystal sugar until evenly coated.
Slice the log into rounds approximately 1/2 inch (1.2 cm) thick.
Arrange the cookies on the prepared baking sheet, leaving a little space between each one.
Bake for 12–15 minutes, until the edges and bottoms are lightly golden while the tops remain pale.
Cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.
