Coat 9-inch springform pan with nonstick cooking spray. Combine chocolate and butter in large glass bowl. Microwave on high 1 minute until melted; stir until smooth. If needed, microwave another 30 seconds to 1 minute; stir. Let cool slightly.
Meanwhile, heat oven to 375°. Knead together almond paste and 3 tablespoons confectioners? sugar on flat work surface. Roll out into 8-inch circle.
Beat granulated sugar into chocolate mixture until smooth, about 1 minute. Add eggs, one at a time, beating after each until blended; beat on high speed until slightly airy, about 2 minutes. Beat in vanilla and salt until blended. Sift cocoa powder over top of chocolate mixture; beat on low speed until blended, about 1 minute. Pour half the batter into prepared pan. Carefully fit almond paste round on top of batter in pan. Scrape remaining chocolate batter into pan; spread to cover round and smooth the top. Gently arrange whole almonds on top of batter.
Bake in 375° oven 30 minutes. Remove pan to wire rack and let cool to room temperature; refrigerate 2 hours. Remove side of pan. Sprinkle top with 1 tablespoon confectioners? sugar. Slice and serve.