Let the butter to room temperature, beat it until fluffy then add in icing sugar. Mix well.

Sift in flour and salt, mix until well-combined. Add in egg, mix well and form a dough. Wrap it with cling film and chill for one hour. 

Divide the dough into 5 pieces. Place one dough on parchment paper, then cover with another parchment paper. Roll it into 1cm thin sheet. 

Pipe the purple sweet potato filling on the pastry, then roll it. Cut it into small pieces and place on lined baking sheet. Brush egg wash on top, then use finger to dip some egg wash, then dip in black sesame seed onto the pastry for decoration. 

Pop in preheated oven at 170C, bake for 25-30 minutes. Let it cool completely on wire rack and then store in air-tight container.