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Preheat oven to 350F
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In a medium saucepan, whisk together 1 cup sugar, flour, cornstarch, and salt. Stir in water and lemon juice.
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Cook over medium-high heat, stirring frequently, until mixture comes to a boil. Stir in butter.
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Place egg yolks in a small bowl and gradually whisk in 1/2 cup of hot sugar mixture. Whisk egg yolk mixture back into remaining sugar mixture.
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Bring to a boil and continue to cook while stirring constantly until thick. Remove from heat.
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Grease a muffin or tart tin and press 1-2 Tablespoons of cookie dough into each indentation and press to shape into a cup shape (along the bottom and sides of the indentation).
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Bake the cookie cups for 8-12 minutes (will vary depending on the size of your tin).
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Set oven to broil.
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Fill each cookie cup with 1-2 Tablespoons of the lemon filling and then top with a Tablespoon of marshmallow fluff.
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Place all of the cookie cups under the broiler for 2 minutes, or until the marshmallow topping appears browned.
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Let cool before serving.
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Cream the butter and sugar together with a mixer until light and fluffy, about 2 minutes.
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Add in the eggs and vanilla.
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In a separate bowl, stir together the flour and baking powder and then slowly add it to the sugar-butter mixture.
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Once all of the flour is incorporated into the dough, form into a ball and cover with plastic wrap.
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Refrigerate for 30 minutes before shaping into cookie cups.