Preheat oven: Preheat oven to 350°F (175°C). Grease a 9x9-inch baking pan.

Make the streusel: In a bowl, combine 0.5 cups brown sugar, packed (streusel) brown sugar, 0.3 cups all-purpose flour (streusel) flour, and 1.5 teaspoons ground cinnamon (streusel) cinnamon. Cut in 3 tablespoons cold unsalted butter, cubed (streusel) cold butter with a fork or your fingers until the mixture resembles coarse crumbs. Toss in 0.3 cups chopped pecans or walnuts (streusel, optional) nuts if using. Set aside.

Cream butter and sugar: Beat 0.5 cups unsalted butter, softened softened butter and 0.8 cups granulated sugar sugar together until light and fluffy, about 2-3 minutes.

Add wet ingredients: Beat in 2 large eggs eggs one at a time, then mix in 1 cups cashew milk yogurt cashew milk yogurt and 1 teaspoons vanilla extract vanilla until smooth.

Combine dry ingredients: In a separate bowl, whisk together 2 cups all-purpose flour flour, 1 teaspoons baking powder baking powder, 0.5 teaspoons baking soda baking soda, and 0.5 teaspoons fine salt salt.

Mix batter: Add the dry ingredients to the wet mixture in two additions, stirring just until combined. Do not overmix.

Assemble: Spread half the batter into the prepared pan. Sprinkle half the streusel over it. Spread the remaining batter on top, then finish with the rest of the streusel.

Bake: Bake until a toothpick inserted in the center comes out clean, about 35-40 minutes.

Cool: Let the cake cool in the pan for at least 15 minutes before slicing.