Preheat oven to 350°F. Grease a 9x13 inch baking pan or line with parchment paper.

In a large bowl, whisk together flour, sugar, cocoa powder, baking soda, baking powder, and salt until no lumps remain.

Add eggs, buttermilk, hot coffee, oil, and vanilla. Beat on medium speed for 2 minutes until smooth. Batter will be thin.

Pour batter into prepared pan. Bake for 30-40 minutes until toothpick inserted in center comes out clean.

Cool cake completely in pan on wire rack before adding mousse.

For mousse, combine mini marshmallows, chocolate chips, and milk in heavy saucepan over low heat. Stir constantly until melted and smooth.

Remove from heat and cool to lukewarm, stirring occasionally.

In separate bowl, whip cold heavy cream until stiff peaks form.

Gently fold cooled chocolate mixture into whipped cream until no white streaks remain.

Spread mousse evenly over cooled cake. Refrigerate for at least 4 hours or overnight before slicing.

Slice with sharp knife dipped in hot water. Serve chilled.