Preheat the oven to 350°F (175°C). Grease a 9x9-inch baking pan or line with parchment paper.

Make the streusel: In a small bowl, whisk together flour, brown sugar, and cinnamon. Cut in cold butter using a pastry blender or your fingertips until mixture resembles coarse crumbs. Stir in walnuts. Set aside.

Make the cake: In a medium bowl, whisk together flour, baking powder, baking soda, and salt.

In a large bowl, beat softened butter and granulated sugar with an electric mixer on medium speed until light and fluffy, about 2 minutes. Scrape down the bowl.

Add eggs one at a time, beating well after each addition. Mix in vanilla.

In a small bowl or measuring cup, stir together Greek yogurt and milk until smooth.

With the mixer on low speed, add the flour mixture in three additions, alternating with the yogurt mixture in two additions, beginning and ending with flour. Mix just until combined — do not overmix. The batter will be thick.

Spread half the batter into the prepared pan. Sprinkle with half the streusel. Spread the remaining batter on top (it helps to drop spoonfuls and gently spread). Sprinkle the remaining streusel evenly over the top.

Bake for 30-35 minutes, until a toothpick inserted in the center comes out clean or with a few moist crumbs. The top should be golden brown and the edges just pulling away from the pan.

Cool in the pan on a wire rack for at least 20 minutes before slicing. Serve warm or at room temperature.