Gianduja or gianduia is a homogeneous blend of chocolate with 30% hazelnut paste, invented in Turin during Napoleon's regency (1796–1814). It can be consumed in the form of bars or as a filling for chocolates.

Gianduja is chocolate stretched with hazelnut butter. Similarly to standard chocolate, it is made in both plain and milk versions. It may also contain other nuts, such as almond. As a bar, gianduja resembles normal chocolate, except for the fact that it is softer due to the presence of hazelnut oil, which is liquid at room temperature unlike cocoa butter. However, like conventional chocolate, gianduja is usually tempered.

The Continental System, imposed by Napoleon in 1806, prevented British goods from entering European ports under French control, putting a strain on cocoa supplies. It is unclear when gianduja bars were made for the first time. However, Kohler is generally credited for the addition of (whole) hazelnuts to chocolate bars in 1830. It is also known that, in 1852, Turin-based chocolate manufacturer Caffarel invented gianduiotto, which is a small ingot-shaped gianduja.

It takes its name from Gianduja, a Carnival and marionette character who represents the archetypal Piedmontese, natives of the Italian region where hazelnut confectionery is common. A textbook by Le Cordon Bleu writes this naming was an attempt to appeal to children.

Gianduja is a simple blend of chocolate, powdered sugar, and finely ground nuts, generally hazelnut. In gianduja made on an industrial scale, these nuts are almost always almonds and hazelnuts.

To begin, nuts are toasted to develop flavor and reduce moisture, as water can impact viscosity. In nuts intended for white chocolate gianduja, toasting is brief. While still hot, the nuts are then refined in a melanger or food processor until pieces are around the size of cocoa, leaving some small pieces to prevent the final gianduja being too soft. A food processor creates a less smooth product, but in domestic and some artisan production it is the most accessible option. During refining, oil is released from the nut's cells; the addition of some powdered sugar and the nuts residual heat increases the output.

This nut paste is then added to the melted chocolate with the rest of the powdered sugar. All chocolates—milk, white, and dark—are used to make gianduja. The ratio of chocolate to nuts determines the firmness of gianduja; using more chocolate creates a softer gianduja. In gianduja intended as a confection filling, the ratio of chocolate, sugar and nuts are around 1:1:1. Depending on the chocolate used, other ingredients may include lecithin, milk fats and vegetable fats. It can be made in about 30 minutes.

Gianduja quickly dries when exposed to air, so manufacturers often coat it with chocolate.