To make the cake, start by creaming the butter and sugar in a large mixing bowl.
Add the egg, vanilla extract, and lemon juice to the butter and sugar mixture, mixing well.
Combine the flour, baking powder, and salt in a separate mixing bowl.
Add the dry ingredients to the creamed mixture, alternating with milk and mixing 'til smooth.
Next, gently fold in the blueberries with a spatula, taking care not to "smoosh" them up.
Once everything is mixed together well, spread the coffee cake batter into the 8x8" baking pan that you greased up in the first step.
For the streusel topping, combine the cold salted butter, brown sugar, flour and cinnamon in a small mixing bowl with a pastry cutter, 'til the mixture resembles bread crumbs with a few large chunks of butter here and there.
Once it's all mixed together, crumble your streusel topping over the cake batter in the cake pan.
Bake your cake at 375ºF for 35-45 minutes, or 'til a toothpick or cake tester inserted into the center comes out clean.
Serve and enjoy with a piping hot cup of coffee or tea!