1. Coat bottom and sides of an 8-inch-square baking pan with cooking spray. Line bottom and two sides with parchment, leaving a 2-inch overhang. In a large bowl, stir together 1 1/2 cups peanut butter, 1 stick butter, and vanilla until smooth. Stir in confectioners' sugar, 1/2 cup at a time, until smooth. Transfer to prepared pan; press with your hands to flatten and smooth top (if mixture is too sticky, dampen them slightly).

2. Combine chocolate and 4 tablespoons butter in a metal bowl. Set over a saucepan containing 2 inches of simmering water; stir with a rubber spatula until chocolate has melted and mixture is smooth. Remove bowl from heat and let cool slightly, stirring gently, about 3 minutes. Pour chocolate over peanut-butter mixture in baking pan, tilting pan to ensure chocolate spreads evenly across top.

3. Meanwhile, in a small bowl, stir together remaining 2 tablespoons butter and 1/4 cup peanut butter until smooth. Let stand until slightly thickened, about 5 minutes. Drop 1/4-to-3/4-teaspoon rounds of mixture on top of chocolate in baking pan in 1-inch intervals. Drag the tip of a wooden skewer or toothpick quickly through center of each round to create a heart shape. Refrigerate until firm, at least 4 hours or, loosely covered, up to 1 week.

4. Run a paring knife along edges of pan on the two sides not lined with parchment. Use parchment overhangs to lift from pan; cut into 2-inch bars. Serve cold.