Heat oven to 375°F. Grease a 9 x 13-inch baking dish. Line with enough parchment paper or foil so it is hanging at least 1-inch over the sides of the dish and grease the parchment or foil.

Place cookies in the bowl of a food processor fitted with the blade attachment, breaking them up as you go. Pulse until cookies are in crumbs. Add sugar, butter, and salt, and process until combined.

Transfer mixture to the prepared baking pan and press evenly on to the bottom and sides of the dish, making sure the crust comes up at least 1 inch on all sides.

Bake on the middle rack until fragrant and browned, about 15 minutes. Set on a wire rack to cool slightly and lower oven temperature to 325°F.

Beat eggs until smooth in a large bowl. Add condensed milk, grapefruit juice, lemon juice, and grapefruit zest. Whisk until smooth.

Pour filling into crust and bake until just set, but still a bit wobbly in the center, about 30-40 minutes (bars will firm up once chilled). Let cool, then refrigerated for at least 2 hours.

Before serving, combine cream, lavender, and powdered sugar in the bowl of a stand mixer fitted with the whisk attachment. Whisk until stiff peaks form. Using the parchment paper, gently lift the bars out of the baking dish and spread cream over the top of the bars, slice and serve.