Preheat the oven to 350ºF (180°C). Line the bottom of a round 9-inch (24cm) springform pan (or springform cake tin) with parchment paper and spray or butter the sides of the pan.
Mix polenta, flour, baking powder, and salt in a bowl.
Beat ricotta and water until smooth in a large bowl with an electric mixer. If it's still somewhat curdled, don't worry.
Add sugar, honey, and lemon zest and beat well.
Gradually add melted butter and then add eggs, one at a time, and beat until the mixture is smooth.
Add the dry ingredients in two or three additions, beating on low speed only until incorporated.
Pour the batter into the pan, dot the top with the tablespoon of chilled butter, and bake for 35 to 45 minutes, until the top is golden and a cake tester or toothpick inserted in the middle comes out clean.
Let cool completely on a wire rack and remove from the tin or pan onto a serving plate, and serve with the blueberry sauce. You can also sprinkle the top with powdered sugar (known as icing sugar in some countries) before serving if you like a sweeter cake.