You don’t have to worry about the type of flour you use for these brownies too terribly much. I love oat flour and it has such a nice, soft texture but you can certainly use all-purpose flour if that’s what you have on hand. I didn’t use nuts in these, but you certainly could. I’d start with 3/4 cup and see how you feel about the ratio, adding a few more if you’d like.
Preheat oven to 400 degrees F. Line an 8-inch-square baking pan with parchment paper (or spray the pan well with nonstick cooking spray).
In the top of a double boiler set over barely simmering water, melt the butter and chocolate together. Stir often, and remove from heat when it’s smooth.
In a larger bowl, whisk together sugar, cocoa, espresso powder and salt. Pour in the melted chocolate mixture and stir well. Whisk in eggs one at a time, followed by the egg yolk and vanilla. You want to stir the batter pretty vigorously here to make sure everything’s incorporated.
Carefully fold in the flour and stir until no dry bits remain.
Scrape batter into prepared pan and bake 20 minutes, or until the center seems set (doesn’t jiggle; be careful not to overbake – they firm up as they cool).
Allow brownies to cool completely in pan, then refrigerate until bottom of pan is very cold, at least one hour. Slice and enjoy!