Whoopie Pies with Mint Filling and Chocolate Ganache

COOKIES:
Position rack in center of oven and preheat to 375°F. Line 2 large rimmed baking sheets with parchment paper.

Sift flour, cocoa powder, baking powder, and baking soda into medium bowl.

Using electric mixer, beat butter, sugar, egg yolks, and vanilla in large bowl until well blended, about 2 minutes. Add flour mixture into the creamed mixture in 3 additions adding it alternately with milk in 2 additions, beating until blended after each addition.

Drop dough by very rounded tablespoonfuls onto prepared baking sheets, spacing 3 inches apart. Bake 1 sheet at a time until cookies are slightly puffed and spread but are still soft, 8 to 10 minutes. Cool on sheets 10 minutes. Carefully transfer cookies to racks and cool completely. Repeat with remaining cookie dough.

Note: The cookies can be made 1 day ahead. Store airtight in single layer at room temperature and assemble them the next day.

MINT FILLING:
Using electric mixer, beat butter, peppermint extract, and vanilla in large metal bowl until blended. Add powdered sugar, egg whites, and pinch of salt; beat until light and fluffy. Add food coloring drop by drop for desired shade of green; beat until well blended. If mixture looks curdled, place bowl over low heat several seconds and beat until smooth. Repeat as necessary until smooth.

CHOCOLATE GANACHE:
Bring cream to simmer in a heavy small saucepan. Remove from heat. Add chocolate; whisk until smooth. Cool until lukewarm but spreadable, about 15 minutes.

ASSEMBLE:
Spoon 1 teaspoon of ganache onto bottom (flat side) of half of the cookies. Place cookies, ganache side up, on prepared baking sheet.

Spoon mint filling into a pastry bag fitted with medium star tip (or spoon filling into a resealable plastic bag, squeezing filling into 1 corner of bag, then cut off 1/4 inch of plastic bag corner to allow for piping). Starting at the outer edge of the bottom (flat side) of the remaining cookies and working toward center, pipe mint filling in a spiral. Place 1 mint-covered cookie, mint side down, atop each ganache-topped cookie, pressing slightly to adhere.

DO AHEAD: Can be made 2 hours ahead. Store airtight at room temperature.