In a food processor, pulse vanilla shortbread until finely ground, stopping to scrape down the sides of the bowl as needed, about 15 pulses. Transfer the crumbs to a large bowl. In a small microwave-safe bowl, microwave the butter on high, stirring occasionally, until melted, about 1 minute. Add the melted butter to the crumbs and stir until the mixture resembles wet sand. Press evenly into the bottom of a non-stick 9-inch tart pan with a removable bottom. Freeze for at least 15 minutes.
While the crust chills, finely grate the lemon zest on a microplane to yield 4 teaspoons. Then, juice the lemons and measure ½ cup of juice. In a medium nonreactive bowl, add the sweetened condensed milk, lemon juice, and, 3 teaspoons zest. Whisk the filling until smooth and starting to thicken, about 1 minute. Pour the filling onto the chilled crust and spread into an even layer. Cover and refrigerate until set, for at least 2 hours or up to 12 hours.
Place the whipped cream in a piping bag fitted with a Wilton 1M tip. Carefully remove the sides and bottom of the tart pan and place the tart on a cardboard cake round or large serving plate. Pipe a circle of swirls around the perimeter of the tart. Sprinkle reserved teaspoon lemon zest over the whipped cream. Refrigerate in an airtight container for up to 4 days.
Yield 8 servings
Tips/Notes
I prefer to use Walker's Vanilla Shortbread cookies, but you can also use graham crackers, Golden Oreos, or Nilla Wafers.
I used full-fat sweetened condensed milk. You can substitute fat-free sweetened condensed milk, and the flavor will remain just as delicious. However, because fat plays a structural role in helping no-bake chocolate fillings set, using a fat-free version can make the filling set slightly softer than standard condensed milk.