Preheat oven to 350°F. Line a 9-inch x 9-inch baking pan with parchment paper and set aside.
Melt butter in a medium saucepan over medium heat. Remove from heat and stir in brown sugar, salt, baking powder and vanilla.
Add eggs one at a time, whisking until smooth and combined. Stir in flour until just combined. Fold in coconut. Transfer batter to prepared pan and smooth surface. Sprinkle with flaky salt.
Bake 35-40 minutes, or until edges are set and top is shiny and golden brown, but middle is still soft. Let cool completely, then slice into squares.