Preheat your oven to 400°F (200°C). Line a large baking sheet with parchment paper.

Gently unfold your thawed puff pastry sheet onto a lightly floured surface. If it’s a bit thick, use a rolling pin to roll it out into a rectangle that's about 10x12 inches.

Spread the mixed berry jam evenly over the entire surface of the puff pastry, leaving a small half-inch border along one of the long edges.

Carefully fold the pastry in half, bringing the long edge without jam over to meet the long edge with the jam border. Gently press down to seal the edges together.

Use a sharp knife or a pizza cutter to slice the folded pastry into 1-inch wide strips (about 12 strips). Take one strip and gently hold both ends. Twist it several times in opposite directions to create a spiral shape. Place on the prepared baking sheet. Repeat with the remaining strips, leaving space between each.

In a small bowl, beat the egg with a splash of water for egg wash. Brush lightly over each pastry twist and sprinkle generously with granulated sugar.

Bake for 15 to 20 minutes until puffed up, golden brown, and filling is bubbly.

Let the twists cool on the baking sheet for a few minutes before transferring to a wire rack to cool completely. Meanwhile, whisk together powdered sugar, milk, and vanilla extract to make the glaze until smooth and drizzly. Drizzle over cooled twists and let set for 15 minutes before serving.