• Preheat the oven to 350F / 175C and line an 8" x 8" baking tin with parchment paper. 

For the base

• Crush the graham crackers in a food processor, blender or by hand into fine crumbs. Place in a small microwave-safe dish with the butter and cook in the microwave for 30 seconds until the butter has melted.
• Stir the butter into the graham cracker crumbs until totally absorbed. Tip the buttery crumbs out into the lined baking dish and pat them down to form a thin base. Set aside. 

For the brownie layer

• Place the chocolate and butter into a large microwave-safe bowl and cook in the microwave for 30-60 seconds until fully melted. Stir to combine. Stir in the yoghurt and eggs until everything is totally blended.
• Mix in the flour, cocoa powder, sugar, baking powder and salt until the mixture just comes together and there are no pockets of dry ingredients. Pour on top of the graham cracker base and spread into the corners. 
• Bake in the preheated oven for 20 minutes. When cooked, remove from the oven and leave to cool slightly on a wire rack for 10 minutes. 

For the topping

• Slice each marshmallow into 4  equal pieces and place them evenly on top of the warm brownie so they stick. Use a kitchen blowtorch to brown the marshmallows. Alternatively, place the brownie under a preheated grill for 2 minutes, until the marshmallows have melted and turned golden brown. 
• Slice the brownies into 9 even squares and enjoy straight away while warm.