1 and 1/4 cups all-purpose flour, plus more for dusting
1 and 1/2 tsp. coarse salt
1 stick cold unsalted butter, cut into cubes
4 oz. cold cream cheese, cut into cubes
1/2 cup plus 1 tablespoon ice water
1 cup raspberry jam
1 large egg, lightly beaten
1/2 cup powdered sugar
1 to 2 tbsp. whole milk
Pulse flours and salt in a food processor to combine. Add butter and cream cheese and pulse until small pieces remain. Add ice water and pulse until dough just starts to clump together.
Turn out dough (it will be crumbly) onto a clean work surface and divide in half. Wrap each in a piece of plastic wrap and press to form a flat rectangle. Refrigerate dough until firm, at least 20 minutes and up to 1 day.
Preheat oven to 375 degrees. Roll out each rectangle on a lightly floured piece of parchment to a 10-by-14 1/2-inch rectangle. Transfer one rectangle (with parchment) to a baking sheet. Spread jam on top, leaving a 1-inch border all around. Roll up remaining rectangle on rolling pin, then unroll on top of first rectangle. Press edges together, then use a crimped pastry wheel to trim all around. Brush beaten egg over top, then use a paring knife to cut a few vents in top crust. Refrigerate 30 minutes.
Bake until golden and jam is bubbling up through vents, about 35 minutes. Let cool completely on baking sheet on a wire rack. Pastry can be stored, covered in plastic, up to 1 day before glazing and cutting.
Whisk together powdered sugar and 1 tablespoon milk in a small bowl; if mixture is too thick to pour, add more milk, a drop at a time. Drizzle icing over top of pastry, then let set at least 10 minutes before cutting into 12 squares, 3 across short side, 4 down long side.