streusel:.

Combine brown sugar, flour, and cinnamon in medium bowl. Add melted butter; toss with fork to blend. Using fingertips, rub mixture together until small clumps form. Mix in pecans. DO AHEAD Can be made 1 day ahead. Cover and chill.

cake:.

Position rack in center of oven and preheat to 350°F Butter 9x9x2-inch metal baking pan. Combine flour, baking soda, nutmeg, and salt in medium bowl; whisk to blend. Using electric mixer, beat brown sugar, butter, and vanilla in large bowl until well blended, about 2 minutes. Add eggs 1 at a time, beating well after each addition and scraping down sides of bowl occasionally. Add half of flour mixture; beat just until blended. Add yogurt; beat just until blended. Beat in remaining flour mixture just until blended.

Spoon half of batter into prepared baking pan; spread evenly. Sprinkle half of streusel evenly over batter. Spoon remaining batter in dollops over streusel, then spread evenly over with offset spatula. Sprinkle remaining streusel evenly over top.

Bake cake until streusel topping is brown and tester inserted into center of cake comes out clean, about 45 minutes. Cool cake in pan on rack 30 minutes. Cut into squares and serve slightly warm or at room temperature.