Cream the butter, then add the flour and whipping cream. Mix until everything is well combined. Wrap the dough in plastic wrap and chill for 45 minutes.

Preheat the oven to 350°F.

Take the dough out of the fridge and divide it into two parts. Roll out one piece at a time to about ⅛-inch thickness. Cut into 1 ½-inch rounds, dip each side in sugar, and place them on a parchment-lined baking sheet. Prick the cookies with a fork to avoid bubbles and bake until puffed and crisp, about 7-10 minutes.

Let the cookies cool. Continue baking the rest of the dough and prepare the filling while the cookies cool.

Combine the rhubarb, lemon juice, lemon zest, sugar, cornstarch, and salt in a pot. Bring to a boil, then reduce to a simmer. Cook until the mixture thickens and has a jam-like consistency, about 10-15 minutes.

Remove from heat and let cool for about 45 minutes.

Once the rhubarb mixture is cool, whip the butter until smooth. Add the rhubarb jam, powdered sugar, and vanilla extract. Whip until smooth, adding more sugar if needed to thicken.

To assemble the cookies, spread 2 teaspoons of filling on the bottom of one cooled cookie, then top with another cookie to form a sandwich.