Preheat your oven to 350°F (175°C). This ensures that your cookie cups bake evenly.
Prepare the crust: In a bowl, combine the graham cracker crumbs and melted butter until the mixture resembles wet sand. This will form the delicious base of your cookie cups.
Form the crusts: Press the graham cracker mixture into the bottom of a muffin tin to create individual crusts, about 1/4 inch thick.
Bake the crusts: Place the muffin tin in the preheated oven and bake for 8 minutes or until the crusts are lightly golden. Remove them from the oven and allow them to cool.
Make the filling: In a separate bowl, whisk together the sweetened condensed milk, fresh lemon juice, and egg yolks until smooth and well combined. This mixture will create a creamy lemon filling.
Fill the crusts: Pour the lemon mixture into the cooled crusts, filling each cup almost to the top.
Bake the filling: Return the muffin tin to the oven and bake for 12-15 minutes or until the lemon filling is set but still slightly jiggly in the center.
Cool and chill: Allow the lemon cups to cool to room temperature, then refrigerate for at least 2 hours. This chilling step helps the flavors meld together beautifully.
Top and serve: Once chilled, top each lemon cup with a spoonful of raspberry preserves. Garnish with lemon zest and a pinch of sea salt before serving.