Chocolate Dough: Cream butter and sugar with electric mixer fitted with paddle attachment on low speed until well blended, scraping side of bowl occasionally. Stir in egg until combined.
Sift flour, cocoa and salt; add to butter mixture. Blend well. Scrape side of bowl; mix an additional 10 seconds. Transfer to a piece of parchment paper and flatten to about 1-inch thickness. Wrap in paper and refrigerate at least 1-1/2 hours.
Roll dough on lightly floured surface to fit 10-inch fluted tart pan with removable bottom. Press into bottom and up side of pan; dock bottom of dough. Refrigerate 20 min. or until firm.
Bake in 350°F standard oven (or 325°F convection oven) 18 to 20 min. or until crust is firm. Cool completely in pan on wire rack.
Chocolate Filling: Bring half of the milk and half of the sugar to boil in heavy saucepan on medium heat, stirring occasionally. Whisk remaining milk and sugar, egg and cornstarch in bowl. Whisk a small amount of the hot milk mixture into egg mixture to temper. Add egg mixture to saucepan. Bring to boil 1 min., whisking constantly. Stir in chopped chocolate until melted.
Pour chocolate mixture into bowl of electric mixer fitted with paddle attachment. Mix on low speed 5 to 8 min. or until chocolate mixture has cooled and steam subsides.
Chocolate Brownie Streusel: Combine ingredients with electric mixer fitted with paddle attachment on low speed until crumbly. Transfer to parchment paper-lined sheet pan. Bake in 325°F convection oven 10 to 12 min. or until crisp. Cool completely in pan on wire rack.
For each tart: Pour cooled filling into prepared crust. Sprinkle with OREO Medium Cookie Pieces, gently pushing into filling. Refrigerate at least 2 hours or until set.
Sprinkle OREO Brownie Streusel onto cooled tart. Cut into 12 pieces.