Preheat the oven to 400F (205C). Line baking sheets with parchment paper and set aside. Sift the flour, cream of tartar, baking soda, and salt into a small bowl.

In a large bowl, combine the butter, shortening, and 1½ cups (375 mL) sugar. Beat thoroughly with an electric mixer on medium speed until light and fluffy, about two minutes. Add eggs and beat until creamy and well combined. Add the dry ingredients and beat until combined. In a small bowl, stir the remaining ¼ cup (65 mL) of sugar with the cinnamon.

Shape the dough into 1½-inch balls (1 tablespoon/15 mL per ball) and roll in the cinnamon sugar. Arrange the balls approximately 2 inches (5 cm) apart on baking sheets. Bake two sheets at a time until the edges of the cookies are set but the centres are still soft (they will not brown), eight to 10 minutes, rotating the sheets halfway through. Transfer the sheets to wire racks to cool for five minutes. Repeat with the remaining dough balls. Store in an airtight container for up to a week.

Yield: 24 cookies.