Serve slices of this cake along with a cup of Ceylon black tea. Light and subtle, the tea's honey and citrus flavors make it a natural foil for a variety of fruits, buttery cakes, and custard desserts such as creme brulee.
Make the cake: Preheat oven to 350 degrees. Coat a 4 1/2-by 9-inch loaf pan with cooking spray, line with parchment, and lightly coat again. Stir together flours, yeast, 3/4 teaspoon salt, and the flaxseed. In a separate bowl, whisk together yogurt and butter.
Beat together eggs, yolks, granulated sugar, and vanilla with an electric mixer on medium speed until pale and thick, about 6 minutes. With a large rubber spatula, gently fold in flour mixture until incorporated. Gently fold in butter mixture until incorporated, then fold in raspberries (do not overmix). Pour batter into pan.
Bake until pale gold and a toothpick inserted into center comes out clean, 45 to 50 minutes, rotating halfway through. Let cake cool in pan on a wire rack 20 minutes. Remove cake from pan; let cool completely on wire rack.
Make the icing: Melt butter in a small saucepan over medium-low heat. Stir in milk, vanilla, and a pinch of salt. Whisk in confectioners' sugar, 1/4 cup at a time, until icing is thick but pourable (you may not need all the sugar). Pour icing over cake, garnish with raspberries and flaxseed, and let set at least 10 minutes before cutting.