Step 1: Preheat your oven to 350°F (175°C). Grease and flour a 9x5 inch (23x13 cm) loaf pan, or line with parchment paper.
Step 2: In a large bowl, cream together the softened butter and granulated sugar until light and fluffy. This should take about 3-5 minutes with an electric mixer.
Step 3: Add the eggs one at a time, beating well after each addition until fully incorporated. Stir in the vanilla extract.
Step 4: In a separate medium bowl, whisk together the flour, baking powder, and salt.
Step 5: Gradually add the dry ingredients to the wet ingredients, alternating with the milk, beginning and ending with the dry ingredients. Mix until just combined, being careful not to overmix.
Step 6: Divide the batter evenly into two separate bowls.
Step 7: To one bowl, add the cocoa powder and mix until fully combined and the batter is uniformly chocolate-colored.
Step 8: To the second bowl, add the pistachio paste (and optional green food coloring, if using) and mix until the batter is uniformly green.
Step 9: Spoon alternating dollops of chocolate and pistachio batter into the prepared loaf pan. Start with one color, then add the other, continuing until all batter is used.
Step 10: Take a skewer or a butter knife and gently swirl through the batters 2-3 times to create the marble effect. Be careful not to overmix.
Step 11: Bake for 45-50 minutes, or until a wooden skewer inserted into the center of the cake comes out clean. If the top browns too quickly, you can tent it loosely with aluminum foil.
Step 12: Let the cake cool in the pan for 10-15 minutes before inverting it onto a wire rack to cool completely. Slice and serve.