Preheat oven to 350 degrees. Line an 8x8 baking dish (I used a glass dish) with parchment paper.
In a large bowl, mix together melted butter and brown sugar. Mix until creamy and combined.
Add in the egg, vanilla and salt until combined. Next add the flour, about 1/3 cup at a time until all combined. Next, fold in the diced persimmons and gently mix until they are integrated and evenly distributed into the batter.
Bake for 25-30 minutes, making sure a toothpick inserted into the center comes out clean.
Cool the dish on a wire rack. Frost when cool to the touch.
Melt butter in a medium sized saucepan over medium-low heat. Add brown sugar and milk. Stir mixture and bring to a boil. As soon as mixture starts to boil, remove from heat and cool for 5 minutes.
Add in powdered sugar and mix until all lumps disappear.
Pour/spread icing onto cooled blondies. Cut into 12 squares once icing has set and cooled.