1.Preheat oven to 180°C/350°F. Grease 24cm x 32cm (9½-inch x 13-inch) swiss roll pan; line base and long sides with baking paper, extending paper 5cm (2 inches) over sides.
2.Beat butter and sugar in medium bowl with electric mixer until light and fluffy. Beat in eggs, one at a time; stir in ground hazelnuts and sifted flour. Spread mixture into pan; bake about 20 minutes. Stand slice in pan 15 minutes.
3.Meanwhile, stir chocolate and oil in medium heatproof bowl over medium saucepan of simmering water until smooth; spread chocolate mixture over slice. Refrigerate until set before cutting
To achieve a smooth chocolate glaze, spread the glaze over the slice with a spatula, then tap the pan gently on the bench until the glaze becomes smooth. Store squares in an airtight container for up to a week.Note