- Preheat oven to 375°F (190°C).
- Using electric beaters, beat butter until light. Beat in eggs and Parmesan cheese. Using wooden spoon, gradually stir in flour. Transfer dough to floured work surface; knead gently into ball. Dust dough lightly with flour, roll out to ¼” (5 mm) thickness. Using cookie cutter, cut out squares. Remaining dough can be re-rolled once. Transfer to parchment paper-lined baking sheets.
- Prick surface of shortbread with fork. Bake in centre of oven for 12 to 15 minutes until golden. Cool in pan on rack for 5 minutes.
- Transfer to racks to cool completely.
Creamy Mushroom Topping
- In skillet, melt butter over medium heat. Add onions, garlic and basil. Cook, stirring, for 2 minutes. Add mushrooms. Cook, stirring, for 15 to 20 minutes or until excess moisture has evaporated and mushrooms are browned. Turn off heat; stir in sour cream. Spoon on Parmesan shortbread and serve.