Mix together butter, flour, and confectioner’s sugar. Press into the bottom of a 9×13-inch baking pan. Bake for approximately 12-15 minutes, or until lightly golden brown.
Remove from oven and allow to cool completely before filling.
For the key lime layer:
Reduce oven to 325º F.
Whisk egg yolk, sweetened condensed milk, lime juice, lime zest, and lemon juice in a large bowl until well-combined.
Beat egg whites with an electric mixture until they begin to foam. Gradually add in granulated sugar and beat until light and fluffy and soft peaks have formed. Gently fold into egg yolk mixture and carefully spread onto completely cooled shortbread crust.
Bake for 15-20 minutes, until set throughout. Remove from the oven and chill on a wire rack. Lightly cover and place in the refrigerator to chill for at least 8 hours.
For serving, cut into squares and top with whipped cream and lime slices or lime peeling slivers.
For the whipped cream:
Add heavy cream into a large bowl. Beat on high speed until it begins to thicken. Slowly add in confectioner's sugar while beating on high speed. Add vanilla extract and continue beating until stiff peaks form.