Heat the oven to 350 degrees F and butter an 8-inch square baking pan. Cream the butter and 1 cup of the sugar in the bowl of a standing mixer until light and fluffy, 3 to 5 minutes, scraping down the sides of the bowl halfway through. Add the egg and beat until incorporated.
In a small bowl, whisk together the flour, baking powder and salt. Combine the milk with the cream in another bowl. Add half of the dry ingredients to the mixing bowl and beat briefly until just incorporated. Add half of the milk and cream and do the same. Repeat once more, alternating the dry and wet ingredients.
Briefly beat in the lemon juice and zest. If you’re using frozen blueberries, toss them in a tablespoon or so of flour, and then gently fold in the berries with a spatula. Be careful not to over-mix the batter!
Spread the batter evenly in the pan. Combine the remaining sugar with the cinnamon and sprinkle evenly over the top of the cake. Bake for 30 to 40 minutes, until a toothpick comes out almost clean — do NOT overbake! (If this cake is made right, it has a very moist, almost pudding-like texture.) Cool completely in the pan, on a rack. Cut into squares and serve on its own, or with whipped cream or ice cream. The cake will keep for a few days in an airtight container.
