Heat milk in a heavy-bottomed pan or kadai over medium heat, stirring occasionally in between. Bring to a boil.
Reduce the heat to low, and continue cooking the milk. Scrape the sides of the pan and stir frequently to prevent it from sticking at the bottom of the pan. Cook until the milk reduces to half (2 cups).
Now, add sugar, mix well and cook further for 2-3 minutes, until the sugar dissolves.
Add cardamom powder, and half of the slivered almonds and pistachios. Mix well, and cook for another 1-2 minutes.
Switch off the heat, and transfer the basundi to a heat-proof bowl. Set aside to cool down completely.
Once the basundi reaches room temperature, add the custard apple pulp, and mix well. Cover the bowl, and refrigerate for 2-3 hours or until you are ready to serve.
Transfer to serving bowls and garnish with the remaining 1/2 tablespoon each of slivered almonds and pistachios.