There are two important rules for filling macarons: avoid any fillings that are extremely sweet (the macaron shells are very sweet), and don’t overdo the filling or it will squish out when you bite into a macaron.
- LEMON OR CITRUS CURD (about 1 cup)
Combine 1/2 cup lemon juice, strained before measuring, with 6 tablespoons unsalted butter, and 1/2 cup sugar in an enameled iron saucepan. Bring to a boil over medium heat, stirring occasionally. Whisk 6 large egg yolks in a small bowl, then whisk in a third of the boiling liquid. Return liquid to a boil and whisk in the yolk mixture. Decrease the heat to low and continue whisking gently until the curd thickens. Pour into a plastic container or jar, press plastic wrap against the surface, and chill.
Substitute lime or passion fruit juice for the lemon juice.
Or use 1/3 cup orange or tangerine juice and 2 1/2 tablespoons lemon juice.
- GANACHE (about 3/4 cup)
Melt and cool 6 ounces bittersweet or semisweet chocolate. Combine 1/3 cup heavy whipping cream and 1 tablespoon light corn syrup in a small saucepan; whisk together, bring to a simmer, then cool to room temperature. Whisk the cooled cream mixture into the chocolate, then whisk in 1 tablespoon very soft unsalted butter. Chill briefly if too thin to spread.
- CHOCOLATE BUTTER FILLING
Easier than ganache and has better keeping qualities. Melt 6 ounces semisweet or milk chocolate and cool it to room temperature. Whisk in 6 tablespoons (3/4 stick/3 ounces) VERY soft butter. Use right away before it sets.
- JAM OR PRESERVES (about 1 cup)
Combine 1 cup jam or preserves and 2 tablespoons water or rum in a small saucepan. Bring to a simmer over low heat, stirring occasionally. Strain into a bowl, rinse the pan and return the strained mixture to it. Bring to a simmer again and cook until slightly thickened, about 5 minutes. Cool slightly, then use. For advance preparation, reheat and cool before using.
- FILLING MACARONS
First, pair all the shells since there are always slight differences in size. Line up the paired macarons on a paper-covered pan with plenty of room between rows, then separate the pairs. Use a pastry bag, paper cone, or a non-pleated plastic bag to pipe the filling. Hold the bag in one hand and pipe a teaspoon or slightly more of the filling on the flat base of one of the shells, then immediately cover with the other matched shell, gently pressing it into place so that the filling shows between the coupled shells.