Preheat oven to 350° F. Grease a 8" x 8" baking pan and line with parchment paper so that there is an overhang of 2".

In a medium to large saucepan, melt butter, brown sugar, and salt together over low heat, until just melted. Remove from heat and whisk to combine.

Add egg and vanilla to pot, whisking to combine.

Add flour, graham cracker crumbs, and baking powder to saucepan, stirring to combine. Place mixture in baking pan and bake for 20-25 minutes, or until a toothpick comes out with a couple of crumbs on it.

Let cool completely in pan.

Place whipping cream and chocolate together in a small saucepan. Heat over low heat for 4-5 minutes, or until most of the chocolate has melted. Stir together to melt remaining pieces. Remove from heat and set aside too cool.

Set the bowl of a stand mixer over a small saucepan of simmering water with egg whites and sugar in it. Whisk by hand briefly to combine. Heat for about 5-7 minutes, or until egg white and sugar mixture is hot to the touch (or to 140°F if you have a candy thermometer). Carefully transfer bowl to stand mixer, add cream of tartar and beat, using the whisk attachment for 8-10 minutes, or until there is no residual heat left and stiff peaks have formed. Add vanilla and whisk to combine.

Top blondies with ganache and marshmallow fluff. Carefully torch marshmallow fluff if desired. Cut into squares (I found parking the pan in the freezer for a few minutes allowed for cleaner cutting)*.