In a large bowl, warm milk for 30 seconds in the microwave. Mix in yolk, sourdough starter& poolish.
In a separate container, mix flours,salt,sugar, yeast. Mix with wet ingredients using a fork, then wet hands to knead into a shaggy dough.
Move dough to a clean bowl & proof in a warm area for 30 mins.
Stretch & fold the dough, then proof for another 30 mins.
Repeat folding, flatten the dough, wrap & freeze for 3hrs.
Prepare a butter block: Flatten 200g of butter between parchment paper to 0.5-1 cm. Chill in the fridge for 30 mins.
Flatten the dough on a floured surface.
Place butter block in the center, fold the dough to seal the butter & roll out to 1cm thick.
Fold each end toward the center then fold like a book. Freeze for 30 mins.
Roll dough to 1cm thick, fold into thirds, wrap & freeze for 2hrs.
Flatten the dough to 0.5-1 cm thick.
Cut triangles with a sharp knife or pizza cutter.
Roll triangles from base to tip & place on a sheet, tip side down.
Proof for 2 hours or store in an airtight container for up to 1 week (defrost & proof takes 6-8 hours from frozen).
Preheat oven to 220C for 30 mins.
Brush pastries with egg wash.
Place in oven lower to 200C & turn on convection. Bake for 10 minutes, rotate pan, & lower to 190C.
Bake for 7-9 mins more.
