Preheat the oven to 350 degrees.
Grease a 12 cup bundt pan.
In a small mixing bowl whisk together the flour, cornstarch, baking powder and salt. Set aside.
Add the pistachios to the bowl of a food processor and process until very fine, about 1 minute 30 seconds. Set aside
In the bowl of a stand mixer fitted with the paddle attachment combine the mascarpone, sugar, lemon zest and juice. Mix on medium speed until fluffy, about 5 minutes.
Mix in the eggs one at a time. Add in the pistachios and mix until fully incorporated. Lower the speed to low and begin adding in the dry ingredients alternating with the milk making sure you end with the dry ingredients.
Pour into the prepared pan and bake for 45-50 minutes or until a toothpick comes out clean. Cool in pan for 10 minutes then invert the cake onto a plate to make sure it easily comes out. Put the cake back into the pan.
To make the soak and glaze:
For the soak mix together the water, lemon juice and sugar. Pour 1/3 of the mixture over the cake (still in its pan) every 10 minutes. This will ensure the cake absorbs the liquid evenly. 30 minutes total.
To make the glaze whisk together the confectioners sugar, lemon juice and milk or heavy cream. Invert the cake and pour over. Decorate with extra pistachios.