Yield: 1 tart, 4-6 servings.
1 cup all-purpose flour
¼ tsp. salt
6 tbsp. cold unsalted Kerrygold Pure Irish Butter, cut into ½" chunks
1 large egg, separated
3-5 tsp. ice water
2 cups raspberries
3 Tbsp. granulated sugar, plus additional 2 tsp. for crust
2 tsp. cornstarch
optional, powdered sugar
For dough: Heat oven to 425°F. In food processor, combine flour and salt; pulse to combine. Add butter; pulse just until mixture resembles coarse meal. Add egg yolk and water, 1 tsp. at a time; pulse just until moist clumps form. Do not overprocess or allow dough to completely form a ball. Turn dough out onto lightly floured surface; dough will be crumbly. Working with a small amount at a time, use the heel of your hand to smear the dough away from you. With bench scraper, scrape up dough and form a ball. Repeat with remaining dough, adding dough to form one ball. On parchment paper, roll dough to 11" or 12" round; transfer to half-sheet pan.
To shape tart: Arrange raspberries in center of dough, leaving 2" border around edge. In small bowl, mix together 3 Tbsp. granulated sugar and cornstarch; sprinkle evenly over berries. Carefully fold dough up over filling, pleating edge and leaving an opening in center. Gently pinch to seal pleats and any cracks. Lightly beat egg white; brush over folded edge of tart. Sprinkle with 2 tsp. granulated sugar.
Bake on lower oven rack until crust is golden on bottom and juices are bubbling, 25-30 minutes. Cool on pan 15 minutes. Use spatula to loosen tart from parchment (especially where juices have leaked) and transfer to wire rack; cool until warm. Serve warm or at room temperature. Dust with powdered sugar, if desired. To serve, cut in wedges.