Line bottoms of cake pans with parchment paper, and grease the side. Preheat oven to 375 degrees.
In a large saucepan, warm the butter, molasses, and milk together, just until the butter is melted, and ingredients well mixed.
Beat in sugar and eggs.
Add dry ingredients, and beat until well mixed.
Divide cake batter evenly between the two cake pans, and bake just until cakes are well set - about 22-25 minutes.
While cake is cooling, make the frosting:
In a stand mixer, whip mascarpone and powdered sugar until well mixed.
With mixer on slow, gradually add in the heavy cream, until all cream is incorporated into the mascarpone and sugar mixture.
Increase mixer speed to medium high, and beat until frosting is light and fluffy. This happens quickly, about 3 minutes in my mixer.