• Preheat oven to 350ºF (180ºC).

Make the easy streusel:

• In a medium bowl, stir ½ cup brown sugar, ½ cup all-purpose flour, 1 teaspoon ground cinnamon, 2 teaspoons unsweetened cocoa powder and a pinch of salt until combined.
• Scatter 4 tablespoons unsalted butter, cold and in small pieces, and work with your hands or a dough cutter until it's clumpy with no dry spots.
• Add ½ cup coarsely chopped pecans and mix. Reserve. If making ahead, keep refrigerated in an airtight container.

Make the vanilla cake:

• Butter or spray a 10-inch or 10-12 cup tube cake pan (see Notes below) and line the bottom with parchment paper.
• In a large bowl, cream 1 cup unsalted butter, softened, and add 1 ½ cups sugar gradually, beating until creamy, 3 minutes. Use an electric mixer or a stand mixer with the paddle attachment. 
• Add 3 eggs, one at a time, beating well after each one. Add 2 teaspoons vanilla extract and mix to combine.
• Add the sifted dry ingredients: 3 cups all-purpose flour, 2 teaspoons baking powder, ½ teaspoon baking soda, and ½ teaspoon salt in 3 parts, alternating with 1 cup regular sour cream in 2 parts. This means you start and end with flour. You can sift them before or have the ingredients measured and sift them directly over the butter mixture (my choice). 
• Mix well but don't overbeat. The cake batter will be thick and velvety. 

Assemble the coffee cake:

• Put half of this mixture in the prepared pan. Smooth it with the back of a damp spoon (it will glide more easily) and scatter half of the streusel mixture over it.
• Drop mounds of remaining batter mixture by tablespoons and carefully smooth it with the back of a spoon (if it sticks, dampen the spoon again). 
• Sprinkle the remaining streusel over it.
• Bake for 50-60 minutes, until a cake tester or toothpick comes out dry. It may take longer depending on your oven and the pan used.
• Let it cool on a wire rack for 10-15 minutes. Use a flat knife to carefully loosen the edges that might be stuck due to the sugar in the streusel.
• Let cool completely on a wire rack before removing. Place a wire rack on top of the pan and carefully turn it upside down. 
• Transfer to a plate and glaze. 

Make the glaze:

• Mix 1 cup powdered sugar and 2 tablespoons lemon juice in a small-medium bowl until creamy and no lumps remain.
• Pour over the cold cake and let it drip down the sides. Let it set and dry before cutting and serving.
• It keeps for a few days, well wrapped, at room temperature.