Place the berries in large bowl and sprinkle with the sugar and cornstarch, and toss. Sprinkle with the lemon juice and toss again. Transfer them into a shallow 2-quart baking dish.
Roll out the pastry dough onto its parchment paper onto a baking sheet. Cut about 2/3 of the pastry into 1-inch thick strips on the diagonal. Use a small star-shaped cookie cutter to cut stars in the remaining corner of the dough, but leave everything in place (don’t remove any of the pastry from the sheet). Brush the top of the dough lightly with the egg yolk, and sprinkle over the sugar
Place the fruit cobbler and the pastry on the baking sheet in the oven on two separate racks. After about 16 minutes, remove the pastry, when it is golden brown and flaky. Leave the cobbler in the oven for another 10 to 15 minutes, until the filling is bubbling around the edges. Place the baking sheet on a wire rack and allow the pastry to cool a bit. When the cobbler is done cooking, allow that to cool for about 15 minutes on another wire rack.
While the cobbler is baking, make the whipped cream. In a large mixing bowl, using a whisk or a hand mixer, whip the cream and confectioners’ sugar just until slightly firm peaks are formed.
Lift the cooked sweet pastry strips from the paper and place them over the top of the cobbler, trimming them as needed so that they fit neatly into the baking dish. Leave about 1 inch between each pastry strip, so that the berries peek through. When you have covered about 3/4 of the cobbler with the stripes, pop the stars out from the pastry and use those to fill in the remaining space.