Step 1

Make the cake: Preheat the oven to 350˚ F. Lightly butter a 9-inch square baking dish; dust with flour and tap out the excess. Whisk the flour, baking powder, baking soda, poppy seeds and salt in a medium bowl. Beat the butter, granulated sugar and lemon zest in a large bowl with a mixer on medium-high speed until light and fluffy, 3 to 4 minutes. Add the eggs, one at a time, beating well after each addition, then beat in the vanilla. Reduce the mixer speed to low and beat in the flour mixture in two batches, alternating with the sour cream and scraping down the bowl as needed. Spread the batter in the baking dish.

Step 2

Make the topping: Mix the flour, brown sugar, oats, poppy seeds, nutmeg and salt in a bowl; work in the butter with your fingers until the mixture looks like coarse meal.

Step 3

Sprinkle the topping over the batter. Bake, rotating the baking dish halfway through, until a toothpick inserted into the center comes out clean, 50 minutes to 1 hour. Transfer to a rack and let cool completely. Dust with confectioners' sugar just before serving.