Flatten chilled butter between greaseproof paper into an 8x15cm rectangle, 1cm thick, and refrigerate.
Mix room temperature butter, flour, milk powder, caster sugar, salt, and 150ml water in a mixer.
Add yeast and mix for five minutes on low, then seven minutes on medium speed.
Form dough into a ball and let rest in a covered bowl for 15 minutes.
Prove dough in the microwave at 100W for six minutes, then rest for 10 minutes.
Roll dough into an 18x25cm rectangle, 1cm thick.
Freeze dough on a lined baking sheet for 10 minutes.
Place chilled butter on dough, fold in thirds, roll to 18x25cm, and repeat.
Fold dough in thirds and freeze for 15 minutes.
Perform two double-folds (book folds), rolling out between each, and freeze for 15 minutes.
Wrap dough in film and chill in the fridge.
Mix almonds, extract, sugars, powder, and egg yolk into a paste.
Roll paste into twelve 5cm long sausages and chill.
Roll dough to a 40x30cm rectangle and cut into twelve triangles.
Place a filling sausage on each triangle and roll into croissants.
Prove croissants on a lined tray for one hour.
Preheat oven to 220°C.
Brush with beaten egg and bake for 8-10 minutes until golden.
Cool on the tray for five minutes, then move to a wire rack.
Dust with icing sugar once cool.