Bring milk and butter to simmer in heavy medium saucepan.
Remove from heat.
Add chocolate; whisk until melted and smooth.
Transfer to small bowl. Cool. Cover and refrigerate until thick and cold, at least 1 day and up to 3 days.
Place yolks in medium bowl. Bring 4 tablespoons butter and cream to simmer in heavy small saucepan. Gradually whisk hot cream mixture into yolks. Anchor bowl with yolk mixture by placing bowl on wet kitchen towel; set aside.
Stir sugar and 6 tablespoons water in heavy medium saucepan over low heat until sugar is dissolved. Increase heat and boil without stirring until syrup is deep amber color, occasionally brushing down sides of pan with wet pastry brush and swirling pan so caramel will color evenly, about 10 minutes. Slowly whisk hot caramel into yolk mixture, then whisk until smooth.
Transfer caramel mixture to small bowl; let stand until no longer warm to touch (caramel will become too thick if it cools too long), about 1 hour.
Whisk in remaining butter. Cover and chill until thick and cold, at least 1 day and up to 3 days.
Preheat oven to 400°F. Line 2 large baking sheets with parchment paper. Blend powdered sugar and almonds in processor until nuts are ground to powder, scraping sides of bowl often, about 8 minutes.
Add cocoa and blend 1 minute more. Using electric mixer, beat egg whites in large bowl until stiff but not dry. Fold nut mixture into whites in 4 additions, making thick batter.
Spoon half of batter into pastry bag fitted with 1/2-inch plain round tip. Pipe batter onto each prepared sheet in 12 walnut-size mounds, spacing mounds apart (cookies will spread slightly).
Bake cookies, 1 sheet at a time, until firm to touch in center and dry and cracked on top, about 11 minutes. Slide parchment with cookies onto work surface; cool cookies. Repeat with remaining batter, cooling sheets completely and lining with clean parchment for each batch.
Arrange 1 macaroon, flat side up, on work surface. Drop 1 scant tablespoon filling onto cookie. Top with second macaroon, flat side down. Press lightly to adhere, making sandwich. Repeat with remaining macaroons and filling. Arrange macaroons on platter. Cover; chill at least 2 hours and up to 1 day.
