Combine first 4 ingredients, stirring well. Press mixture into bottom of a 9-inch springform pan. Bake at 350° for 15 minutes. Let cool completely on a wire rack.
Beat cream cheese at medium speed of an electric mixer until fluffy. Combine 3/4 cup brown sugar and cornstarch; add to cream cheese mixture, beating well. Add milk and liqueur, beating at low speed until blended. Add 3 eggs, one at a time, beating just until blended. Spoon cream cheese mixture into prepared crust. Bake at 325° for 50 minutes. Remove from oven; increase temperature to 400°.
Combine sour cream, 3 tablespoons sugar, egg yolk, and 1/2 teaspoon vanilla. Spread mixture on top of cheesecake. Bake an additional 5 minutes. Turn oven off. Partially open oven door, and let cheesecake cool in oven 30 minutes. Remove from oven, and let cool on a wire rack. Cover and chill thoroughly. Arrange pecan halves on cheesecake.