Pastry Cream

• The most common filling for traditional French eclairs is pastry cream. It’s a thick, puddinglike custard you must cook and cool before you can use it as filling. To prepare pastry cream, gently simmer egg yolks, sugar and cream, whisking constantly to prevent lumps from forming. Add pure vanilla extract or vanilla beans after the mixture thickens to flavor the filling. Though it may take a little practice to feel confident about making pastry cream, it’s not a complicated task. The constant whisking seems tedious but is otherwise straightforward.

Whipped Cream

• Of the two most common eclair fillings, whipped cream is more common in American eclairs and easier to prepare. Simply add sugar to heavy cream or whipping cream, then let an electric mixer do all the work. The cream needs to hold stiff peaks, but watch for it to start separating -- clear liquid begins to pool in the whipped cream if it's separating. Whisking cream by hand is an unparalleled workout for your upper arms. The taste of fresh whipped cream is well worth any effort you put in. You can flavor it with vanilla extract, but some bakers prefer the natural taste of sweetened cream. Whipped cream is prone to melting if you expose it to even a little heat. Most bakeries stabilize their whipped cream with unflavored gelatin, which is why bakery whipped cream seems extra-firm or stiff.

Flavored Fillings

• While vanilla is the most common filling flavor, French bakeries often sell eclairs with chocolate- and coffee-flavored fillings. Many bakeries frequently use fruits common in French desserts, such as raspberry and orange, in eclair fillings. But, less traditional flavors also make an appearance. Passion fruit, pistachio, dulce de leche, lemon, green tea and liquor-spiked pastry creams also fill eclairs. Adding fruit or nut extract or fruit juice to pastry cream is relatively simple as long as the flavored ingredient is liquid or otherwise fine in texture. Adding an extra ingredient to whipped cream can be impossible unless it is a finely milled powder, like cocoa, or an extract. If you choose to flavor your filling, it is safer to use pastry cream rather than whipped cream.

Filling Methods

• Choux pastry is mostly hollow once it has been baked, making it easy to pipe filling into it. Fit a piping bag with a metal decorating tip, and scoop the filling into the bag. Gently insert the metal tip into one end of the oblong pastry and squeeze until filling starts to surround the tip. Alternatively, you can slice along a long side of the pastry, open it like a hot dog bun, and spoon or pipe filling into it. If you cut the pastry, handle it gently: It’s hollow, flaky and delicate.