Prep a small mixing bowl by adding the cubed butter to it, along with a fine mesh sieve. Set aside.
Add the sugar, lemon juice, lemon zest, egg yolks, and salt to a small saucepan.
Set the saucepan over medium heat, stirring constantly (this can be slowly - we just need constant movement happening!)
Continue to stir the curd until it thickens and reaches a temperature of at least 170°F / 76°C.
Remove the curd from the heat and pass it through the fine mesh sieve, over the butter. Mix the curd and the butter together until the butter has melted and the mixture is smooth.
Transfer the curd to the fridge to chill until you're ready to assemble the cookies.
Once the cookies have chilled, preheat the oven to 350°F / 175°C. Line a cookie sheet with parchment paper or with a silicone baking mat.
Use a 3 tbsp. cookie scoop to dish out the cookies onto the prepared cookie sheet, evenly spacing them apart to allow room for spreading.
Bake for 10-12 minutes, until the sides of the cookies just start to turn golden brown, and the top of the cookies looks just slightly wet. Let the cookies rest on the hot baking sheet for at least 5 minutes before transferring them to a wire rack to cool.
While the cookies come to room temperature, make the swiss meringue.