Preheat oven to 350°F (175°C). Grease and flour a 9-inch round or 8×8-inch square pan.

1. In a small bowl, toss diced rhubarb with 1 tablespoon sugar. Set aside.
2. In a medium bowl, whisk together flour, baking powder, baking soda, salt, and cinnamon.
3. In a large bowl, beat eggs with granulated sugar, then add melted butter, milk or buttermilk, and vanilla extract. Stir until smooth.
4. Gently fold dry ingredients into wet ingredients just until combined.
5. Fold in sugared rhubarb until evenly distributed in the batter.
6. Pour batter into prepared pan and smooth the top.
7. If using streusel, combine streusel ingredients in a small bowl until crumbly, then sprinkle evenly over the batter.
8. Bake for 45–55 minutes, or until a toothpick inserted in the center comes out clean or with moist crumbs.
9. Cool cake in pan for 15 minutes, then transfer to a wire rack to cool completely.
10. If using glaze, whisk powdered sugar with lemon juice or milk and drizzle over the cooled cake before serving.