That lemon filling, so exclusive in my experience to doughnuts and commercial bakery products, is actually quite easy to make at home. This lemon filling uses cornstarch and gelatin to create its thick texture, and egg yolks for body and color. It's a little quicker than a lemon curd, which requires longer cooking, and it sets up firmer too.
This is a great filling for pre-baked tartlets and sandwiched between cookies. Be on the lookout for a recipe later this week that shows off this filling to yummy, gooey effect.
1 0.25 ounce packet unflavored gelatin
1 1/2 cups warm water
2 egg yolks
1 1/2 cups sugar
3 tablespoons cornstarch
1 tablespoon butter
1/3 cup fresh-squeezed lemon juice
In a medium bowl, sprinkle the gelatin over the warm water and let it sit for a few minutes to soften. Beat the egg yolks in another small bowl and set aside.
In a medium saucepan combine the sugar and cornstarch, then gradually whisk in the water and gelatin. Heat over medium heat, stirring constantly, until it thickens and boils. Boil for about a minute, then remove from heat and pour a little into the egg yolks. Beat together then gradually add back into the gelatin mixture. Return to the heat and simmer for about a minute. Remove from the heat and beat in butter and lemon juice until smooth.
This will keep for a few days in the fridge.