Generously coat a 3-cup fluted tube pan with baking spray. In a medium bowl beat butter with a mixer on medium 30 seconds. Add sugar, baking powder, and salt. Beat on medium 1 to 2 minutes more or until well combined. Add eggs, one at a time, beating well after each addition. Beat in vanilla. Add flour; beat just until combined. In a small bowl beat mascarpone cheese until smooth. Spoon half the batter into mascarpone cheese. Beat just until combined; set aside. In a small bowl whisk together boiling water and espresso powder until dissolved. Add to remaining plain batter. Stir in melted chocolate. Alternately drop spoonfuls of chocolate and mascarpone batters into the prepared pan. Using a table knife, gently cut through batters to swirl them together (do not overmix). Prepare as directed in desired cooker, below.