Stir the flour, brown sugar, cinnamon and salt in a small bowl. Drizzle the melted butter over the dry ingredients, and stir until well combined (fingers work exceptionally well for this). Mix in chopped nuts.
The streusel should feel clumpy not sandy when gently squeezed between your fingertips. If it seems dry, add more melted butter.
For the cake
Preheat the oven to 350 degrees. Lightly butter the bottom and sides of an 8-inch square or round metal cake pan.
Whisk the sour cream, vanilla and almond extract in a small bowl.
Cream the butter, sugar and eggs in a large bowl with a whisk. Mix until the sugar begins to dissolve.
Sift together the flour, baking powder, baking soda and salt in a medium bowl.
Add half the flour mixture into the butter, eggs and sugar, and lightly mix. Add half the sour cream mixture, and lightly mix. Repeat.
Fold in the lemon zest, and pour into prepared pan. Sprinkle with steusel.
Place in middle of oven, and bake for 30 minutes or until a toothpick comes out clean.