Heat oven to 350 degrees Fahrenheit and grease a 9x9-inch metal baking pan.
Mix brown sugar, cinnamon, and pecans together in a bowl for streusel.
Whisk flour, baking powder, baking soda, and salt together in a separate bowl.
Beat butter and sugar until creamy, then beat in eggs, sourdough discard, and sour cream.
Fold dry ingredients into wet mixture until smooth batter forms.
Spread half the batter into pan, scatter half the streusel, then repeat layers.
Bake 38 to 40 minutes until toothpick inserted in center comes out clean.
Cool in pan 15 minutes before slicing into warm squares.