For the Danishes:
2 large eggs
1 tsp. instant yeast
4 C. all-purpose flour, sifted
1 tsp. pure kosher salt
2 tsp. sugar
1 1/2 tsp. ground cardamom, or ground seeds of 10 cardamom pods
1/2 C. (1 stick) unsalted butter, plus 2 to 3 tsp. softened butter
About 13 C. Rhubarb and Strawberry Jam

For the icing (optional):
1/4 C. confectioners’ sugar, sifted
2 tsp. softened unsalted butter
1/2 tsp. pure vanilla extract
About 2 tsp. warm water
1/4 C. chopped nuts (such as toasted pecans or walnuts, or pistachios)

Make the Danishes: In a medium bowl, beat the eggs, yeast, and % C. warm water together until smooth. Set aside in the refrigerator for 15 minutes to cool. In a large bowl, stir the flour, salt, sugar, and cardamom together, then cut in the 1/2 C. butter until the largest pieces of butter are about the size of peas. Make a well in the center and scrape in the egg mixture. Gradually incorporate the flour mixture into the egg, then turn the dough out onto a floured work surface and knead until smooth and soft but not sticky, about 2 minutes. Clean the bowl, return the dough to the bowl, cover with plastic wrap, and let rise in a warm spot until doubled in volume, about 1 1/2 hours. Preheat the oven to 425°F. Line two baking sheets with parchment paper. Divide the dough into quarters and roll one piece of dough out into a rectangle about 18 inches long and 3 inches wide. Cut into 6 or 8 rough squares. With a sharp knife or pastry wheel, cut a slit from each corner of each square toward the center, stopping about 1/2 inch from the center. Spread a bit of softened butter in the center of each square and spoon 1/2 tsp. the jam into the center of each. Bring every other point (four total) to the center of the pastry and pinch them together tightly over the jam. Using a spatula, carefully transfer the pastries to the baking sheets. Repeat with the remaining dough, butter, and jam. Bake for 8 to 10 minutes, until nicely browned. Let cool completely on wire racks. If desired, make the icing: In a small bowl, whisk all the ingredients together, adding more water if necessary to make an icing that is just pourable. Drizzle it over the pastries (you won’t need all of the icing). Sprinkle the nuts over the pastries and serve.