Step 1

Preheat oven to 350°F. Butter twenty-four 1/3-cup muffin cups. Dust with flour. Sift cake flour, baking soda and baking powder into medium bowl. Beat 2/3 cup sugar and butter in large bowl to blend. Beat in vanilla extract. Add egg yolks 1 at a time, beating well after each addition. Beat mixture until light, fluffy and pale yellow, about 3 minutes. Beat in sour cream and poppy seeds. Mix in dry ingredients. Using clean dry beaters, beat egg whites and cream of tartar in large bowl until foamy. Gradually add remaining 1/2 cup sugar and beat until stiff (but not dry) peaks form. Carefully fold egg white mixture into batter in 2 additions. Gently stir in raspberries.

Step 2

Spoon batter into prepared muffin cups, dividing equally. Bake muffins until golden brown and tester inserted into center comes out clean, about 20 minutes. Transfer muffins to racks. Serve warm or at room temperature. (Muffins can be prepared up to 1 day ahead.) Cool completely. Wrap muffins tightly in plastic and store at room temperature.