In a large mixing bowl, whisk together the instant lemon pudding mix, 3/4 cup of heavy cream, and fresh lemon juice until smooth, about 2–3 minutes. Let this mixture sit for approximately 5 minutes to set.

In a separate bowl, use an electric mixer to beat the remaining 1 cup of heavy cream, powdered sugar, and a pinch of kosher salt. Whip on medium speed for 3–4 minutes until soft peaks form.

Once soft peaks form, gently add the marshmallow crème to the whipped cream and mix on low speed for 1–2 minutes until stiff peaks form.

Carefully fold the marshmallow mixture and the crushed shortbread cookies into the lemon pudding mixture using a spatula.

Divide the mixture into individual serving cups and garnish with additional crushed shortbread cookies and a sprinkle of lemon zest.