MELT butter and chocolate in saucepan on medium heat, stirring frequently. Remove from heat.
BEAT sugar, eggs, corn syrup and vanilla in 6-qt. bowl of mixer fitted with paddle attachment 3 min. Add chocolate mixture; mix well.
SIFT together flour, cocoa powder, baking powder and salt. Add to chocolate mixture; mix well. Pour into parchment paper-lined full-sheet pan (or into lined half-sheet pan for trial recipe).
BAKE in 350ºF standard oven 20 to 25 min. or until top is firm to the touch. Cool completely. Meanwhile, prepare Cheesecake batter.
BEAT cream cheese in 6-qt. bowl of mixer fitted with paddle attachment until creamy. Add sugar; mix until smooth. Add whole eggs and egg yolks, 1 at a time, beating on low speed after each just until blended. Blend in heavy cream and vanilla. Refrigerate until ready to use.
POUR 5 lb. cheesecake batter (or 2.5 lb. batter for trial recipe) over cooled brownie layer in pan. Bake 22 to 28 min. or until cheesecake topping is almost set in center. Cool completely.
CHEESECAKE Brownie Fudge Bars: Cut brownie in full sheet pan into 8x8 (3x2-1/2-inch) pieces to make 64 bars (or cut into 8x4 [3x2-1/2-inch] pieces to make 32 bars for trial recipe). Place in single layer in separate parchment paper-lined full-sheet pan (or in separate lined half-sheet pan for trial recipe). Insert wooden pop stick halfway into thickest side of each brownie, keeping the stick as level as possible. Refrigerate or freeze until ready to dip in melted chocolate.
MELT semi-sweet chocolate in saucepan on low heat, stirring frequently.
DIP bars, 1 at a time, in semi-sweet chocolate, turning to completely coat each bar with chocolate. Slowly pull bar from melted chocolate, gently shaking bar as you pull it to remove excess chocolate. Return bars to prepared sheet pan. (Note: You can also gently scrape underside of each bar before placing on the prepared sheet pan to remove excess chocolate.) Let stand or refrigerate until chocolate coating is firm.
MELT white chocolate in saucepan on low heat, stirring frequently.
ONCE all bars are coated with semi-sweet chocolate, spread each with about 1/2 oz. of the melted white chocolate. Let stand until the white chocolate coating is firm. Wrap individually in plastic wrap. Refrigerate up to 24 hours.