Preheat the oven to 350 °F and spray your baking tin with cooking spray or grease with butter. Line an 8"×8" tin with parchment paper. Make sure there is an overhang of parchment on two sides of your tin to make removing the brownies easier.
Combine the flour, cocoa powder, baking powder and salt in a bowl and set aside.
Melt the butter and chocolate chips in a medium pan over low heat. Make sure you stir everything frequently until the butter and chocolate chips are melted.
Off the heat, pour in the sugar, Irish cream coffee creamer, vanilla and the egg. Mix everything together with a wooden spoon until just combined.
Pour the flour mixture into the pan and fold it into the wet ingredients using a wooden spoon. You’ll want to continue folding until there are no traces of flour left.
Pour the batter into your prepared tin. The batter will be quite thick. Use an offset spatula or the back of a metal spoon to smooth the batter out in the tin.
Bake for 20 minutes. If you insert a toothpick into brownies, it should come out with some moist crumbs on it. The brownies will continue to set in the pan once you bring them out of the oven.
Sprinkle over the remaining chocolate chips, walnuts and marshmallows and pop the tin back into the oven for 5 minutes until the marshmallows are puffy.
Remove from the oven and let the brownies cool. Once they have cooled in the tin for at least 30 minutes, remove them from the tin, using the paperr overhang, and cut into 16 squares.
These brownies will keep at room temperature for 2 days. I wouldn’t suggest putting them in the fridge, as the marshmallows will become too hard.