Preheat oven to 350°F. Prepare Mini Whoopie Pie Pan with vegetable pan spray.
In large bowl, stir together flour, baking powder and salt.
In large bowl, beat butter and sugar with electric mixer until light and fluffy. Add egg whites and vanilla extract and beat until well combined. Alternately add flour mixture and milk in three additions, beating until just combined. Spoon one tablespoon batter into each pan cavity.
Bake 9-11 minutes or until tops of cakes spring back when touched. Cool in pan 3 minutes. Cool completely on cooling grid.
For syrup: combine water, espresso and sugar in small saucepan; heat on medium heat until sugar dissolves, about 3 minutes. Remove from heat; stir in rum. Cool slightly.
*Variation: Omit rum and add 2 additional tablespoons water and 1/4 teaspoon rum extract.
For filling: combine mascarpone, cream, sugar and vanilla in large bowl. Whip with electric mixer until stiff peaks form.
To assemble : Quickly dip one cake in cooled syrup and place in bottom of treat pop. Pipe a swirl of cream mixture from back edge following the curve of the container, then filling in the center. Continue layering until treat pop is full. Top with swirl of cream and cocoa powder or chocolate espresso beans, if desired. Refrigerate at least 1 hour or until firm.