How to Make It
Preheat the oven to 350°. In a large bowl, whisk the flour, cornmeal, sugar and salt. With a pastry blender or 2 knives, cut in the butter until the mixture resembles coarse meal. In a small bowl, beat the egg yolks with the cream; drizzle over the cornmeal mixture. With 2 forks, blend the dough lightly until evenly combined. Using your hands, knead lightly.
On a lightly floured surface, roll out the dough to a 15 1/2-by-10 1/2-inch rectangle. Roll the dough onto the rolling pin, then transfer it to an ungreased 15-by-10-inch jelly-roll pan. Press the dough evenly over the bottom and up the sides of the pan. Freeze the crust until firm, about 30 minutes. Bake the crust for 12 minutes, or until pale and dry.
Spread the walnuts on a baking sheet and bake for 8 minutes, or until lightly toasted. Repeat with the pistachios. Leave the oven on.
In a medium saucepan, combine the sugar and water and cook over moderately high heat, swirling the pan once or twice, until the sugar is dissolved. Simmer over moderate heat, brushing down the sides of the pan with a wet pastry brush from time to time until a deep golden brown caramel forms, about 18 minutes.
Meanwhile, in a small saucepan, bring the cream to a boil with the rosemary and salt. Remove from the heat. Carefully and slowly add the hot cream to the caramel, stirring constantly until smooth. Remove from the heat and stir in the hazelnuts, walnuts and pistachios.
Scrape the nut caramel over the crust and spread evenly with a spatula. Bake the bars in the center of the oven for 25 minutes, or until the topping is golden and bubbling. Let cool.
In a glass bowl, melt the chocolate in a microwave oven at high power, stirring a few times. Let the melted chocolate cool until barely warm, about 15 minutes. Brush the chocolate over the caramel-nut topping. Let the chocolate cool until set, about 1 hour. Cut into 2-by-3-inch bars and serve.