Preheat your oven to 180°/fan 160°C/Gas 4. Line the bottom of a 20cm square tin with parchment, then either butter and flour the bottom and sides, or coat them with a nonstick spray.

Whisk the flour, polenta, baking powder, and salt in a medium bowl, and set aside. Using an electric mixer, beat the butter with sugar in a large bowl until pale and fluffy, for at least 2 minutes. Beat in the eggs one at a time, scrap­ing down the bowl between additions, then add the vanilla and zest. Add a third of the flour mixture, all of the soured cream, and another third of the flour, beating until just blended after each addition. Scrape down sides of the bowl. Mix the remaining third of the flour mixture with the blueberries. Fold the blueberry-flour mixture gently into the cake batter.

Spread the cake batter in the prepared cake tin. Use your original dry-ingredients bowl (see how we look out for your dishpan hands?) to combine the dry topping ingredients with a fork. Mash in the butter with your fork, fingertips, or a pastry blender. Scatter the topping over the batter.

Bake the cake until the top is golden brown and the tester inserted into the centre comes out clean, about 35 minutes. Cool the cake in the tin on a rack for 5 minutes. Run the spatula around the edges of the cake to loosen it, then flip out onto a cooling rack.