LIME CREAM

1. Create a 'double boiler' by placing a medium pot of water over a medium heat, and bringing to a simmer. Place a heatproof glass or stainless steel bowl over the pot, making sure that the bowl does NOT touch the water. 
2. Combine the lime juice, egg yolks, eggs, sugar, and salt in the heatproof bowl, whisking together immediately to stop the sugar from cooking the egg yolks. 
3. Cook the mixture, whisking constantly, until it reaches 180˚f / 80˚c. This may take some time (approx 15 minutes). Be patient, and make sure you do bring it right up to the required temperature. 
4. Remove the bowl from the heat, and set aside, whisking occasionally, until the mixture is 140˚f / 60˚c. Strain through a sieve into a clean mixing bowl. Add the butter, a chunk at a time, blending well with an immersion blender until completely combined before adding the next chunk. Alternatively you can do this in a blender. The cream should be pale and thick by the time you are finished. 
5. Transfer to an airtight container, press a piece of plastic wrap onto the surface of the cream, and cover. Refrigerate overnight, or until ready to use. 

LIME AND PISTACHIO SHORTBREAD

1. In the bowl of a stand mixer fitted with the paddle attachment, beat the butter, sugar, and lime zest until pale and creamy, 2 to 3 minutes. Add the egg yolks and vanilla, and beat until combined. Add the ground pistachios, and mix until incorporated. Add the flour and salt, and mix briefly until a dough forms. 
2. Turn the dough out onto a piece of plastic wrap, and shape into a rough rectangle. Wrap tightly, and refrigerate overnight.
3. When you are ready to bake, preheat the oven to 350˚f / 180˚c. Line two baking sheets with parchment paper. Remove the dough from the fridge, and allow to stand for 5-10 minutes in order for it to be easier to roll. Divide the dough in two, and roll out to approx ¼" or 5mm thick. Cut out 2.5" circles, and arrange on the baking sheets. Re-roll scraps to cut out extra cookies.
4. Bake the cookies for 10-12 minutes, or until set and the edges are beginning to go golden. I tend to do one sheet at a time because my oven can only handle that, but if yours can do two, then you can bake both at once - you may have to adjust the baking time slightly, and rotate the pans half way through.
5. Let the cookies cool for 10 minutes on the baking sheet, before removing to a cooling rack and allowing to cool completely. 

ASSEMBLY

1. Match up the cookies into equal sized pairs.
2. Place the cream into a piping bag fitted with a large round tip (alternatively you can use an offset spatula to spread the cream). 
3. Pipe a blob of lemon cream onto the bottom of one cookie, and sandwich with a second cookie.