Preheat the oven to 350°F (175°C). Grease a 9-inch round or square cake pan and line with parchment paper.

In a bowl, whisk together flour, baking powder, baking soda, and salt. Set aside.

In a large bowl, beat softened butter and sugar until light and fluffy, about 3 minutes.

Add eggs one at a time, beating well after each addition. Stir in vanilla extract.

Gradually add the dry ingredients to the butter mixture, alternating with sour cream. Start and end with the dry ingredients. Mix until just combined.

Gently fold in the chopped pistachios.

In a separate bowl, combine flour, chopped pistachios, brown sugar, and cinnamon. Add the cold, cubed butter and use your fingers or a pastry cutter to mix until crumbly.

Pour half of the cake batter into the prepared pan. Sprinkle half of the streusel mixture over the batter. Add the remaining cake batter and top with the remaining streusel.

Bake for 50-55 minutes, or until a toothpick inserted into the center comes out clean.

Let the cake cool in the pan for 10 minutes before transferring to a wire rack to cool completely.