To Prepare the Filling:
Bring the milk and butter to a simmer in a heavy medium-sized saucepan. Remove from heat, then add the chocolate.
Whisk the mixture until the chocolate melts and is smooth, then transfer it to a small bowl and set aside to cool. When cool, cover and refrigerate until thick and cold, at least 1 day.
May be refrigerated for up to 3 days.
To Prepare the Macaroons:
Blend the powdered sugar and almonds in a food processor until the nuts are ground to powder, scraping the sides of the bowl often. It should take about 8 minutes. Add the cocoa and blend for about 1 minute longer.
Using a whisk or electric beater, beat the egg whites in a large bowl until they are stiff but not dry. Fold in the nut mixture in 4 additions, until it forms a thick batter.
Spoon half of the batter into a pastry bag fitted with a 1/2-inch plain round tip.
Pipe the batter onto each paper-lined baking sheet in 12 walnut-sized mounds, spacing the mounds apart slightly, as they will spread a little as they bake.
Bake the cookies, 1 sheet at a time, for about 11 minutes, or until they are firm to the touch in the center and dry and cracked on top.
Slide the parchment paper with the cookies onto a work surface to cool.
Repeat with the remaining batter, cooling the cookies completely and lining the baking sheets with new paper each time.
Drop about 1 tablespoon of filling onto each cookie, then top with a second cookie flat (bottom) side down (facing and pressing against the filling).
Repeat with remaining macaroons and filling. Arrange on a platter, cover and refrigerate for at least 2 hours. Serve cold.
Makes 24 sandwich cookies
PER COOKIE: 225 calories, 4 g protein, 28 g carbohydrate, 11 g fat (3 g saturated), 8 mg cholesterol, 21 mg sodium, 2 g fiber.