To prepare filling: Line baking sheets with wax paper.
In a bowl of an electric mixer on medium speed, beat butter until creamy, about 1 minute. Add powdered sugar, vanilla extract, egg yolks and cocoa powder, and mix until smooth (scraping down sides of bowl as necessary), about 2 minutes. Using a small knife, spread about 1 tablespoon filling on top of each cookie, making a rounded top. Transfer cookies to prepared baking sheets and refrigerate for at least 1 hour.
To prepare chocolate coating: Break chocolate into small pieces. In a double boiler over gently simmering water (or in a microwave oven), melt chocolate. Whisk in 2 teaspoons vegetable oil (or melted butter), adding more to reach desired consistency. Let mixture cool for a few minutes, then dip tops of cookies into chocolate mixture, holding onto almond base. Return cookies to prepared baking sheets and refrigerate until chocolate hardens, at least 30 minutes. Store in refrigerator and serve chilled.
Makes about 3 dozen cookies.
Note: Our 2007 winner is from Eileen Troxel of St. Paul, who adapted the recipe from Traditional Home magazine.