Step 1: Preheat your oven to 350°F (175°C) and grease a 9-inch round cake pan or line it with parchment paper to ensure easy removal of your cardamom orange cake with pistachio streusel.
Step 2: Prepare the pistachio streusel by combining the chopped pistachios, brown sugar, 0.25 cup flour, and cold butter in a small bowl. Use your fingers or a fork to rub the butter in until the mixture forms coarse crumbs, then set aside.
Step 3: In a medium bowl, whisk together the flour, baking powder, ground cardamom, and salt. Ensure the cardamom is evenly distributed for a consistent flavor in every bite of the cardamom orange cake with pistachio streusel.
Step 4: In a large mixing bowl, cream the softened butter and granulated sugar together until light and fluffy. Add the eggs one at a time, beating well after each addition, then stir in the orange zest and orange juice.
Step 5: Gradually add the dry ingredients to the wet ingredients, alternating with the milk. Start and end with the flour mixture, mixing just until combined.
Step 6: Pour the batter into the prepared pan and smooth the top. Evenly sprinkle the pistachio streusel over the top of the batter.
Step 7: Bake for 45-55 minutes, or until a toothpick inserted into the center of the cardamom orange cake with pistachio streusel comes out clean.
Step 8: Allow the cake to cool in the pan for 15 minutes before transferring to a wire rack to cool completely.