Follow these detailed steps to bake your perfect vanilla birthday cupcakes and prepare the luscious vanilla buttercream frosting.

Make the Cupcakes:

1. Preheat the Oven: Set your oven to 320°F (160°C) and line a cupcake pan with 12 cupcake liners.
2. Prepare Dry Ingredients: In a medium bowl, whisk together the all-purpose flour, baking soda, baking powder, and salt. Set aside.
3. Cream Butter and Sugar: In a large mixing bowl, beat the softened butter and granulated sugar together for about 3 minutes until the mixture becomes light and creamy.
4. Add Eggs: Incorporate the eggs one at a time, mixing on low speed until each egg is fully combined with the butter and sugar mixture.
5. Combine Ingredients: Gradually add half of the dry flour mixture to the wet ingredients. Mix in the yogurt, vegetable oil, and vanilla extract until just combined.
6. Fold in Flour: Gently fold in the remaining flour mixture using a spatula, being careful not to overmix.
7. Fill and Bake: Evenly fill the cupcake liners with the batter. Bake for 20-22 minutes, or until a toothpick inserted into the center of a cupcake comes out clean.
8. Cool: Allow the cupcakes to cool in the pan for about 5 minutes, then transfer them to a wire rack to cool completely.

Make the Buttercream:

1. Beat the Butter: In a mixing bowl, beat the softened butter for about 4 minutes until it becomes fluffy and pale.
2. Add Sugar and Vanilla: Gradually add the powdered sugar, vanilla extract, and milk in two parts, mixing on low speed until well blended.
3. Final Mix: Scrape down the sides of the bowl and mix on low speed for another 2 minutes to ensure everything is fully incorporated.
4. Decorate: Once the cupcakes are completely cooled, pipe the buttercream frosting onto each cupcake. Add sprinkles or other decorations as desired.