Increase the oven temperature to 350ºF (180ºC).
Butter or spray an 8-inch square baking pan. Line it with parchment paper for easier removal. See Notes below.
Melt 6 ounces semisweet chocolate and ½ cup unsalted butter in the microwave in 10-second spurts, whisking every time, or over a saucepan of boiling water (be careful the water doesn't touch the bowl).
In the bowl of an electric mixer fitted with the whisk attachment (or a hand-held electric mixer), beat 1 cup granulated sugar, 2 large eggs and ¼ teaspoon salt until thick and light, 2-3 minutes.
Add 1 cup all-purpose flour in two parts, mixing on low until combined. You can do this part with a spatula too.
Remove the bowl from the mixer and stir in the melted chocolate mixture.
Stir in the raisin mixture, walnuts, and remaining 2 tablespoons whisky.
Pour brownie batter into the prepared pan and spread to even out.
Bake for 20 to 25 minutes, until barely jiggling in the center. A cake tester or toothpick inserted in the center should come out with very moist crumbs.
Cool brownies in the pan on a wire rack, wrap in plastic and refrigerate.
Before serving, sift a fine layer of 2 tablespoons unsweetened cocoa powder over the surface if you want to, brushing off the excess cocoa for a velvety finish.
cut into squares (easier if they are cold from the fridge) and serve.