Prepare the crust by mixing together flour and sugar. Using a pastry blender, 2 knives, or a food processor, cut the butter into flour mixture until you get a bowl of coarse crumbs (this step is like making pie dough, so click Pie Crust link below if you're new to this). Add just enough milk, a tablespoon at a time, until the mixture begins to hold together without being wet. Refrigerate crust for 10-15 minutes before baking.

bake crust about 10 minutes -- just until set and barely beginning to brown.

Prepare filling by combining butter, brown sugar, corn syrup and milk. Bring to a boil over medium heat, continue boiling, uncovered, for about 3-4 minutes. Remove from heat and stir in pecans and vanilla. Spread the pecan mixture evenly over the baked crust. Return to the oven and bake for about 20 minutes -- until the edges are brown and the filling is bubbly and deep brown.

Cool on a wire rack for 4-5 minutes before sprinkling the chocolate chips over the top. Let the chocolate melt slightly, then use a knife or offset spatula to spread the chocolate over the top surface -- don't worry if it's not even, there should be some toffee showing through. Sprinkle the chocolate with remaining chopped pecans.