1. Preheat the oven to 350°F.
2. In a medium saucepan, whisk together the sugar, flour, cornstarch, and salt. Stir in the water and lemon juice.
3. Cook over medium-high heat, stirring frequently, until the mixture comes to a boil. Stir in the butter.
4. Place the egg yolks in a small bowl and gradually whisk in ½ cup of the hot sugar mixture. Whisk the egg yolk mixture back into the remaining sugar mixture.
5. Bring to a boil and continue cooking, stirring constantly, until thick. Remove from heat.
6. Grease a muffin or tart tin. Press 1–2 tbsp of cookie dough into each cup, shaping it along the bottom and up the sides to form a cup.
7. Bake the cookie cups for 8–12 minutes (time will vary with the size of your tin).
8. Set the oven to broil.
9. Fill each cookie cup with 1–2 tbsp of the lemon filling, then top with 1 tbsp of marshmallow fluff.
10. Place the cookie cups under the broiler for 2 minutes, or until the marshmallow topping is browned.
11. Let cool before serving.

From-scratch sugar cookies (optional):

1. Cream the butter and sugar with a mixer until light and fluffy, about 2 minutes.
2. Add the egg and vanilla.
3. In a separate bowl, stir together the flour and baking powder, then slowly add it to the butter mixture.
4. Once the flour is incorporated, form the dough into a ball and cover with plastic wrap.
5. Refrigerate for 30 minutes before shaping into cookie cups.