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1
Preheat the oven to 325F.
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2
Line the bottom of an 8-inch square baking pan with parchment paper and liberally coat the paper and the pan sides with flavorless vegetable oil.
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3
Put 3 ounces of the chocolate and the butter in a medium stainless-steel bowl and set over a pot of simmering water.
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4
Heat, stirring occasionally, until the chocolate and butter melt and are fully combined and the mixture is smooth.
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5
Lift the bowl from the pot.
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6
Set aside.
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7
Sift the flour and salt together into a bowl.
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8
In another bowl, combine the eggs and vanilla extract and whisk together by hand until blended.
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9
Whisk in the sugar.
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10
Whisk the egg mixture into the chocolate.
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11
Add the flour and the remaining 2 1/2 ounces chocolate to the batter and, using a rubber spatula, mix well.
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12
Then mix in the walnuts.
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13
Pour the batter into the prepared pan.
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14
Spread it evenly with a small offset spatula.
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15
Scatter the marshmallow pieces evenly over the surface and push them halfway into the batter.
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16
The tops should remain uncovered.
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17
Bake on the middle shelf of the oven until the marshmallows are browned and a skewer inserted into the center of the brownie sheet comes out with some batter clinging to it, about 45 minutes.
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18
Let cool completely in the pan on a wire rack, then cover with plastic wrap and refrigerate until cold.
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19
Run a table knife around the edge of the pan to loosen the sides of the brownie, and then slide the brownie, still on the paper, onto a work surface.
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20
Using a ruler to guide you and a sharp knife, cut into sixteen 2-inch squares.
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21
Store in an airtight container in the refrigerator for up to 3 weeks.
