Position an oven rack in the center of the oven. Heat oven to 350°F. Butter an 8" cake pan. Place the cake pan on a piece of parchment paper, and trace the pan. With a pair of scissors, cut the circle out, about 1/4" smaller than the tracing. Place the parchment circle in the cake pan, and butter it too. This will make it much easier to remove the cake following baking, and is worth the extra prepping time.
In a medium bowl, combine the flour, cornmeal, sugar, baking powder and salt. Whisk until well blended.
Measure the buttermilk using a 2 cup glass measuring cup (if you don’t have one, measure the buttermilk and pour into a small bowl). Add the egg, oil, vanilla extract, whatever combination of ricotta cheese and cream cheese you choose, and the lemon zest. Mix with a table fork or small whisk until well blended. Pour the liquid over the dry ingredients and whisk together.
Pour the batter into the prepared baking pan.
Sprinkle the blueberries evenly over the top. Place the baking pan on a baking sheet to catch any drips as the cake rises in the oven.
Combine all of the streusel ingredients in a small bowl and mix together with a pastry blender, fork or your fingers until the butter is completely coated and broken down into pea sized pieces. Sprinkle over the top of the batter.
Bake for 40 - 45 minutes, until the top is a pale golden color, and a pick inserted in the center comes out clean.
Move to a wire rack and let cool for 15 minutes. Carefully run a sharp knife around the edges of the pan, and flip the cake out onto a plate. The cake will be upside down, so flip it again back onto the wire cooling rack. Note: much if not all of the streusel will have sunk into the cake creating little ribbons of streusel in the cake.
Serve immediately or cool completely and stow in an air-tight container at room temperature for up to 2 days. We agree with Abby that we liked it best warm so please reheat if needed before serving.