Preheat your oven to 350°F (175°C). Grease and flour a 9x13 inch baking pan, or spray with baking spray.

In a medium bowl, combine 1/2 cup granulated sugar, 1/2 cup brown sugar, 1 1/2 teaspoons ground cinnamon, and 1 1/2 cups all-purpose flour. Add the cold, diced butter. Use a pastry blender, two knives, or your fingertips to cut the butter into the flour mixture until it resembles coarse crumbs. Stir in the chopped pecans, if using. Set aside.

In a separate medium bowl, whisk together 2 3/4 cups all-purpose flour, 1 teaspoon baking powder, 1 teaspoon baking soda, and 1/2 teaspoon salt. Set aside.

In a large bowl, using an electric mixer, cream together 1 cup softened unsalted butter and 1 1/2 cups granulated sugar on medium-high speed until light and fluffy, about 2-3 minutes.

Beat in the eggs one at a time, mixing well after each addition. Stir in 1 teaspoon vanilla extract.

Reduce mixer speed to low. Gradually add the dry ingredients to the wet ingredients in three additions, alternating with the sour cream in two additions, beginning and ending with the dry ingredients. Mix until just combined, being careful not to overmix.

Spread half of the coffee cake batter evenly into the prepared baking pan. Sprinkle half of the streusel topping over the batter.

Carefully spoon the remaining batter over the streusel layer, spreading gently to cover. Sprinkle the remaining streusel topping evenly over the top.

Bake for 50-60 minutes, or until a wooden skewer or toothpick inserted into the center comes out with moist crumbs, not wet batter. If the top browns too quickly, you can loosely tent it with aluminum foil.

Remove the cake from the oven and let it cool in the pan on a wire rack for at least 30 minutes before glazing.

While the cake cools, whisk together 1 1/2 cups powdered sugar, 2-3 tablespoons milk (starting with 2 and adding more if needed), and 1/2 teaspoon vanilla extract in a small bowl until smooth and drizzly. If too thick, add more milk a teaspoon at a time; if too thin, add more powdered sugar.

Once the cake has cooled slightly but is still warm, drizzle the vanilla glaze generously over the top. Allow the glaze to set for a few minutes before slicing and serving. This cake is delicious served warm or at room temperature.