Homemade Chocolate Filled Thumbprint Cookies

Ingredients

• 1 cup (225 g) butter or margarine, softened
• 2/3 cup (130 g) granulated sugar
• 2 egg yolks (large eggs)
• 1/2 teaspoon (2.5 ml) pure vanilla extract
• 2 1/4 cups (290 g) all purpose flour
• Non-stick cooking spray or parchment paper
• For Chocolate Frosting
• 1/2 cup (90 g) semi-sweet chocolate chips
• 1 tablespoon (15 ml) butter or margarine