"I don’t know what else to say other than “YUM, YUM, YUM”. Soooo good. Butter/brown sugar mixtures are my downfall, I had to keep myself from eating the whole pan. :)"
To make the streusel: In a small bowl, mix flour, granulated sugar, brown sugar, cinnamon and salt until well combined. Add the melted butter and mix well until crumbs form. Set aside.