Whisk the flour and salt in a 3-quart / 3 L bowl. Rub in the cold butter with your fingertips until the mixture looks like coarse crumbs with some pea-size flecks left. Stir in the sour cream and egg yolk, then press everything into a shaggy dough, adding 1 to 2 tablespoons cold water only if it won't come together.

Divide the dough in two, flatten each half into a rectangle, and wrap tightly. Chill for 60 minutes, until firm.

While the dough chills, mix the ground almonds, powdered sugar, and cardamom in a bowl. Stir in the melted butter and 2 tablespoons milk until you have a thick paste that holds its shape when pressed. Set aside at room temperature.

Heat the oven to 190 C / 375 F and line a sheet pan, 33 x 46 cm / 13 x 18 inch, with parchment. Roll each dough half on its own sheet of parchment into a rectangle about 24 x 30 cm / 9.5 x 12 inch and 4 mm thick. Slide the first sheet, with its parchment, onto the pan.

Spread the filling evenly over the first sheet, leaving a 1 cm border. Lift the second sheet by its parchment, flip it over the filling, and peel the paper away. Press the outer edges together firmly. Whisk the remaining egg yolk with 1 tablespoon milk and brush it over the top.

Using a sharp knife, cut the sheet into diamonds about 6 cm wide, trimming the uneven edges first, which gives roughly 16 pieces. Press the cut edges of each diamond together with your fingertips. Open edges let the almond paste leak and scorch on the pan. Pull the diamonds apart by about 1 cm.

Bake for 28 to 30 minutes, rotating the pan after 15 minutes, until the tops and edges are deep golden.

Cool the pastries on the pan for 10 minutes. Dust with powdered sugar through a fine-mesh sieve and serve warm or at room temperature.