Prepare Ricotta Filling. Place in a bowl; cover and chill.
Combine 1/2 cup sugar, oil, and egg in a large bowl; beat at medium speed of a mixer until thick and pale (about 5 minutes). Add yogurt and vanilla; beat well. Combine flour, cocoa, baking powder, and salt; stir well. Add to egg mixture; beat well. Set batter aside.
Beat 3 egg whites (at room temperature) and cream of tartar at high speed of mixer until foamy. Gradually add 1/4 cup sugar, 1 tablespoon at a time, beating until stiff peaks form. Gently stir one-fourth of egg white mixture into batter; gently fold in remaining egg white mixture.
Pour batter into a 9 x 5-inch loaf pan coated with cooking spray. Bake at 350° for 40 minutes or until a wooden pick inserted in center comes out clean; let cool in pan 10 minutes. Remove from pan; let cool completely on a wire rack.
Using a serrated knife, split cake into thirds horizontally. Brush each of 4 cut sides of cake with 1 tablespoon Kahlúa. Place bottom layer, cut side up, on a serving platter; spread with 1 cup Ricotta Filling. Arrange 3/4 cup strawberries over Ricotta Filling, and top with middle cake layer. Spread cake with remaining Ricotta Filling, and arrange remaining strawberries over Ricotta Filling; top with the remaining cake layer. Cover and chill for 24 hours. To serve, sift powdered sugar over top of cake. Garnish with mint sprigs, if desired.