Preheat oven to 350 degrees Fahrenheit and grease a 12-count mini bundt pan thoroughly with non-stick baking spray containing flour.
In a large mixing bowl, cream the softened butter and granulated sugar together until light and fluffy, approximately 3 to 5 minutes.
Beat in the eggs one at a time, ensuring each is fully incorporated before adding the next, then stir in the vanilla extract.
In a separate medium bowl, whisk together the flour, baking powder, baking soda, and salt.
Gradually add the dry ingredients to the butter mixture, alternating with the milk, beginning and ending with the dry ingredients; mix until just combined.
Divide the base batter into three equal portions in separate bowls.
Maintain the first bowl as Vanilla; in the second bowl, fold in the cocoa powder to create Chocolate; in the third bowl, fold in the lemon zest and lemon juice for Lemon.
Fill the prepared mini bundt cavities approximately two-thirds full with the respective flavored batters.
Bake for 18 to 22 minutes, or until a toothpick inserted into the center of a cake comes out clean.
Allow the cakes to cool in the pan for 10 minutes before inverting them onto a wire rack to cool completely before glazing.