Pre-heat oven to 350 degrees. Butter a 10-inch loaf pan and dust with flour.
Cream butter with 1 1/3 cups of the flour until light and fluffy. Beat in Creme de Menthe and black peppercorns until well-incorporated.
In a bowl, blend 8 of the egg whites, the salt and the baking powder. Beat until the egg whites hold soft peaks. Continue beating, adding 1 1/3 cups of sugar, a tablespoon at a time, until the egg whites are very stiff.
Stir 1/4 of the egg white mixture into the creamed butter-flour mixture, blending thoroughly with a wooden spoon. Pour this mixture back over the remaining egg whites. Start sprinkling in the remaining 1 1/3 cups of flour while gently folding the butter-flour mixture into the egg whites. The less the mixture is manipulated, the lighter the cake will be.
Pour into prepared pan. Bake for 40 minutes. While baking, mix together the pistachio nuts, the 2 remaining egg whites and the remaining 4 tablespoons of sugar. Remove the loaf from the oven after 30 minutes. Spread the pistachio nut mixture over the top. If the loaf has cracked at all on top, let the pistachio mixture seep into the cracks. If the loaf hasn't cracked, make a few shallow slits on top to let the pistachio mixture seep in. Continue baking until a cake tester comes out clean, and the loaf is starting to pull away from the pan (about 20-30 minutes). Turn out on a rack and let cool for one hour.
The lightness, dryness and low-sweetness of this pound cake is perfect for lightly sweet Champagne.