For the dip: Stir together the sour cream, yogurt and lemon zest in a bowl. Set aside in the fridge.
For the streusel topping: Add the flour, brown sugar, butter, cinnamon and 1 cup of the walnuts to a microwave-safe bowl and stir until combined. Microwave, stopping every minute to stir, until the mixture darkens and starts to come together, about 4 minutes. Cool the streusel flat on a plate until slightly hardened, 5 to 8 minutes.
Add half of the dip to a large bowl, then sprinkle on the chocolate chips followed by a layer of the streusel. Add the remaining dip and smooth the top. Add the remaining streusel and 1 cup walnuts.