In a saucepan, melt butter with hot espresso and cocoa powder. Stir until smooth and glossy.
Add sugar and mix until dissolved. Set aside to cool slightly.
In a large bowl, whisk together flour, baking soda, baking powder, and salt.
Pour the warm espresso mixture into the dry ingredients and mix to combine.
Add eggs, sour cream, and vanilla. Mix until the batter is thick and velvety.
Spread the batter into a greased baking pan and bake at 350°F (175°C) for about 25–30 minutes, or until just set in the center (the cake should remain soft and slightly gooey).
2️⃣ Add the Marshmallow Layer
While the cake is still warm, scatter mini marshmallows over the top of the cake.
Let them soften and melt into the surface of the cake.
3️⃣ Make the Mud Glaze
In a saucepan, melt butter with cocoa powder and espresso over medium heat.
Add sifted powdered sugar and vanilla extract, whisking until smooth and pourable.
Pour the warm glaze over the softened marshmallows, letting it melt and swirl into them to create a gooey, shiny “mud” topping.
4️⃣ Serve
Let the cake cool slightly before slicing.
Slice and enjoy while still warm, allowing the marshmallow mud to stretch with every piece.
