Melt butter with 2/3 cup vanilla chips in top of double boiler over low heat, stirring constantly until smooth. Remove from heat; set aside.
In larger bowl of electric mixer, beat eggs, sugar, and salt at high speed until very pale and thick, about 5 minutes. Blend in melted mixture and vanilla at low speed. Mix in flour until combined. Stir in remaining vanilla chips and cappuccino chips. Spread mixture into prepared pan.
Bake 35-40 minutes or until toothpick inserted into center comes out almost clean. Top will be crusty and browned. Cover pan with foil if browning too quickly. Cool before cutting into squares.