How to Make It
Preheat oven to 350°.
Combine first 3 ingredients; set aside.
Lightly spoon flour into dry measuring cups, and level with a knife. Combine flour, 1/2 cup granulated sugar, baking powder, baking soda, and salt in a large bowl. Combine yogurt, margarine, vanilla, and egg; add to flour mixture, stirring just until moist.
Spread half of cake batter into an 8-inch square cake pan coated with cooking spray, and sprinkle with half of espresso mixture. Top with remaining batter, spreading to cover; sprinkle with remaining espresso mixture. Swirl batters together using a knife, and sprinkle with walnuts. Bake at 350° for 35 minutes or until cake springs back when touched lightly in center. Cool on a wire rack.
Combine milk and 1 teaspoon espresso, stirring until coffee granules dissolve, and stir in the powdered sugar. Drizzle espresso glaze over cake.