Plum Crumble Bars aka "The PLUMBLE" – Flavourfull

Ingredients

• ½ cup (113 g) unsalted butter, cut into pieces
• 1 cup (135 g) all-purpose flour
• 3 tbsp (40 g) granulated sugar
• ½ tsp Diamond Crystal kosher salt
• 1 egg yolk
• 454 g small firm-ripe blue prune plums (about 15)
• 4 tsp tapioca starch
• 3 tbsp (38 g) light brown sugar
• ¼ cup (34 g) all-purpose flour
• 2 tbsp (30 g) unsalted butter
• ¼ tsp Diamond Crystal kosher salt
• ¼ tsp Burlap & Barrel Royal cinnamon
• ¼ tsp Burlap & Barrel Cloud Forest cardamom
• 3 tbsp (20 g) finely chopped toasted walnuts