First, make the crust. Preheat the oven to 325F.

In a medium bowl, whisk together flour and cornstarch.

In the bowl of a stand mixer (at medium speed), beat butter and powdered sugar until light and fluffy. Add in vanilla.

Reduce speed to low, and beat in flour mixture until combined. Dump the dough into an ungreased 9-inch tart pan with removable bottom.

To make handling easier, place large piece of plastic wrap over the dough and use your hands to spread and smooth dough over the bottom and up the sides of the tart pan. Discard plastic wrap, and use a fork to poke holes all over the dough.

Cover the tart dough with parchment or aluminum foil, and pour in pie weights.

Bake the tart for 27-30 minutes, removing the foil/pie weights halfway through cooking.

While the crust cooks, make the filling.

In a medium non-stick saucepan, heat butter and chocolate chips over low heat, stirring occasionally, until melted.

Remove from heat and stir in the brown sugar.

Add eggs, one at a time, whisking very quickly and vigorously after each addition (to prevent scrambled eggs in your pie :) ).

Stir in flour, vanilla, salt, and nuts until evenly blended. Pour into your prepared shortbread crust.

Bake at 325F for about 30 minutes, until top is just set. Cool for 30 minutes to one hour to serve warm, or cool completely to serve later