Line a 20cm square cake pan with plastic wrap, extending the wrap 5cm above the edges. Heat butter in a medium saucepan over medium heat until melted; cook for 8 minutes, swirling pan occasionally, or until browned. Pour butter into pan. Refrigerate for 1½ hours or until firm.

Meanwhile, make croissant dough. Combine yeast, sugar and milk in a large bowl. Cover; stand in a warm place for 10 minutes or until mixture is frothy. Place flour and salt in a large bowl; rub in butter with your fingertips. Make a well in the centre. Add yeast mixture; using a butter knife, cut the liquid through to bring mixture together. Knead for 2 minutes on a lightly floured surface or until a tacky dough forms. Pat out to a rough square. Enclose in plastic wrap; refrigerate for 1 hour.

Turn croissant dough onto a very lightly floured sheet of baking paper. Roll dough into a 30cm square. Lift cold brown butter from pan; fold plastic wrap over butter to enclose. Use a rolling pin to pound butter until butter is pliable and softened. Unwrap butter. Place butter on dough with the corners of the butter touching the midpoint of each side of the dough. Fold corners of dough over to enclose butter.

Roll dough into a 25cm x 50cm rectangle. Fold dough into thirds by bringing the bottom third over the middle third, then the top third down to cover folded dough. Turn dough a quarter turn to the right. Roll out to a rectangle again; repeat folds, turning and rolling one more time. Cover dough with plastic wrap; refrigerate for 30 minutes. Repeat rolling, folding and refrigerating two more times. Refrigerate for a further 30 minutes.

Cut chocolate into 24, 1cm x 10cm lengths using a sawing action with a finely serrated knife (if chocolate breaks, just divide it into 24 portions). Line two large oven trays with baking paper.

Cut dough in half. Roll each half of dough into a 30cm square. Trim edges to reveal layers. Cut each square in half, then cut each half into three; you will have a total of 12 rectangles, about 10cm x 15cm each.

Place a length of chocolate left of centre on a rectangle (chocolate can go right to the edges). Fold edge of dough over chocolate to meet the centre; place another length of chocolate on other side, then roll dough completely to enclose. Repeat with remaining dough and chocolate. Place seam-side down on trays. Cover loosely with oiled plastic wrap. Stand in a warm place for 1 hour or until dough is puffy and springs back a little
when pressed.

Preheat oven to 200°C. Brush croissants with egg, being careful not to brush cut pastry layers. Stand for 5 minutes then brush again.

Bake croissants for 8 minutes. Reduce oven temperature to 180°C; bake for a further 20 minutes or until puffed and golden.