Step 1

PreparationFor cookies: Place slivered almonds in processor and add 1/4 cup sugar. Grind almonds finely. Using electric mixer, beat butter and remaining 3/4 cup sugar in large bowl until well blended. Beat in egg yolk, vanilla extract, and 1/4 teaspoon salt. Add ground-nut mixture and flour and beat until moist clumps form. Gather dough into 2 balls; flatten each ball into disk. Wrap in plastic and refrigerate at least 1 hour and up to 2 days.

Step 2

Line 2 baking sheets with parchment paper. Roll out half of dough between sheets of waxed paper or parchment paper to scant 1/4-inch thickness, sprinkling dough lightly with flour as needed to keep from sticking and occasionally peeling off top paper to remove wrinkles. Peel off top sheet. Using 2 1/2x1 1/2-inch oval cookie cutter (or cardboard template), cut out cookies. If dough is soft, slide rimless baking sheet under paper and chill 10 minutes to firm. Transfer cookies to prepared sheets, spacing 1/2 inch apart. Gather scraps and reroll dough, cutting out more cookies. Chill cookies on sheets 15 minutes before baking.

Step 3

Position racks in top third and bottom third of oven; preheat to 325°F. Bake cookies 5 minutes. Reverse sheets and bake until cookies begin to color, about 6 minutes. Cool cookies on sheets 5 minutes. Transfer to racks; cool completely. Repeat with remaining dough.

Step 4

For filling: Bring cream to simmer in heavy medium saucepan. Remove from heat; add chocolate and whisk until melted and smooth. Let cool until thick but still spreadable, about 2 hours.

Step 5

Place half of cookies, bottom side up, on work surface. Spoon ganache filling into pastry bag fitted with 1/4-inch plain tip. Leaving 1/4-inch plain border, pipe (or spread) ganache on cookies. Top each with second cookie, bottom side down, pressing to adhere.

Step 6

For garnish: Stir chocolate in top of double boiler over simmering water until smooth. Place almonds in small bowl. Dip end of 1 cookie in chocolate, then in almonds. Transfer to sheet of foil. Repeat with remaining cookies. Let stand until garnish is set. (Can be made 2 days ahead. Store airtight in refrigerator in single layers between sheets of waxed paper.