Step 1: Preheat your oven to 350°F (175°C). Lightly grease and flour a 9x13 inch baking pan.
Step 2: Prepare the Shortbread Crust: In a large bowl, cream together the softened butter and granulated sugar until light and fluffy. Beat in the vanilla extract.
Step 3: In a separate bowl, whisk together the flour and salt. Gradually add the dry ingredients to the wet ingredients, mixing until just combined and a soft dough forms. Press the dough evenly into the bottom of the prepared baking pan.
Step 4: Bake the shortbread crust for 15-18 minutes, or until lightly golden brown. Remove from oven and set aside to cool slightly while you prepare the cake batter.
Step 5: Prepare the Streusel Topping: In a medium bowl, combine the flour, brown sugar, and cinnamon. Add the cold, cubed butter. Using your fingertips or a pastry blender, cut the butter into the dry ingredients until coarse crumbs form. Set aside.
Step 6: Prepare the Coffee Cake Layer: In a large bowl, whisk together the flour, granulated sugar, baking powder, baking soda, salt, and cinnamon.
Step 7: In a separate medium bowl, cream the softened butter until smooth. Beat in the eggs one at a time, then stir in the buttermilk and vanilla extract.
Step 8: Gradually add the wet ingredients to the dry ingredients, mixing on low speed until just combined and smooth. Be careful not to overmix.
Step 9: Pour the coffee cake batter evenly over the partially baked shortbread crust in the pan. Sprinkle the streusel topping generously over the batter.
Step 10: Bake for 40-45 minutes, or until a wooden skewer inserted into the center comes out clean. The streusel should be golden brown and bubbly.
Step 11: Let the cake cool in the pan on a wire rack for at least 30 minutes before slicing and serving. This allows it to set properly.