Preheat the oven to 350°F (180°C). Grease a 9x13-inch sheet pan and line the bottom with parchment paper.

Mix cocoa powder and water – In a small bowl, combine the cocoa powder and boiling water to make a smooth chocolate mixture. Set aside.

Cream the butter and sugar – In a large bowl, beat the buttery spread and granulated sugar until light and fluffy.

Add eggs and chocolate – Beat in the eggs, one at a time, and then stir in the cocoa mixture until well combined.

Add dry ingredients – In a separate bowl, whisk together the flour, baking powder, and salt. Gradually fold the dry ingredients into the wet ingredients until just combined.

Fold in the chocolate chips – Stir in the dark chocolate chips to the batter.

Bake the cake – Pour the batter into the prepared sheet pan, spreading it evenly. Bake for 25–30 minutes or until a toothpick comes out clean. Let it cool in the pan for 10 minutes.

Make the frosting – In a medium bowl, mix the cocoa powder, softened butter, milk, and confectioners' sugar until smooth and creamy. Adjust the milk for your desired frosting consistency.

Frost the cake – Once the cake has cooled, spread the frosting evenly over the top.

Decorate the cake – Sprinkle mini marshmallows and crushed chocolate malt balls on top of the frosted cake for a fun, crunchy, and gooey finish.

Let it cool and serve – Allow the cake to set for about 10 minutes before cutting into squares and serving.