Preheat oven to 350°. Coat the inside of a 13x9-inch baking pan with butter.
In a large saucepan, combine molasses, butter, and brewed coffee; bring to a boil over medium-high heat, stirring constantly. Let cool.
Arrange apples in bottom of prepared baking pan.
In a large bowl, whisk together flour, brown sugar, cinnamon, ginger, nutmeg, mace, baking soda, cloves, allspice, and salt.
Stir sour cream into cooled molasses mixture. Add to flour mixture, stirring well. Stir in eggs until well combined. Stir in chocolate and marshmallows. Pour batter over apples.
Bake until a wooden pick inserted in center comes out clean, 55 to 60 minutes. Let cool in pan for at least 10 minutes before inverting gingerbread onto a cutting surface, apple side up. Cut into pieces. Serve warm or at room temperature.
Serve pieces, apple side up, plain or with vanilla ice cream or whipped cream.
Notes
This makes a big pan of gingerbread! You can cut the recipe in half and use an 8x8-inch square baking pan if you wish.
Recipe by TeaTime Magazine at https://teatimemagazine.com/sweet-suspense-is-served-in-the-newest-death-by-chocolate-mystery-by-sarah-graves/