In a large mixing bowl, whisk together flour, brown sugar and sugar.
Cut the softened butter into small pieces and use your fingers to mix until it resembles fine crumbs.
Remove 1 cup of the mixture and set aside.
Stir in buttermilk, eggs, soda and salt.
Pour batter into pre buttered 9 x 9 pan.
Sprinkle reserved crumb mixture evenly over the top.
Bake at 350° for 40 to 45 minutes, until a toothpick inserted in the center comes out clean.
Remove from oven and cool on wire rack for at least 30minutes.