Preheat the oven to 350 degrees . Line 2 cookie sheets with parchment paper. In a medium, microwavable bowl, melt together 2 cups chocolate chips and 2 tablespoons butter on medium power until almost melted, about 1 1/2 minutes; stir just until smooth.
In a small bowl, whisk together the flour, baking powder and 1/4 teaspoon salt. Using an electric mixer, beat together the eggs, granulated sugar and vanilla at high speed until thickened and fluffy, about 5 minutes. Mix in the melted chocolate at medium speed; whisk in the flour mixture. Stir in the remaining 1/2 cup chocolate chips. Refrigerate the dough until firm enough to scoop, but still soft, 10 to 15 minutes.
Onto each prepared pan, drop 12 rounded tablespoons of dough 2 inches apart. Bake, switching and rotating the pans halfway through baking, until the cookies appear bubbly and still soft, about 15 minutes. Let cool completely.
In a medium bowl, beat together the remaining 2 tablespoons butter and the peanut butter. Sift in the confectioners sugar and the remaining 1/4 teaspoon salt; beat until fluffy. On the flat side of each of 8 cookies, spread about 1 1/2 tablespoons of the peanut butter mixture; sandwich with the remaining cookies.