Preheat the oven to 350°F (175°C).
Prepare a 12x18 inch baking sheet by greasing it.
In a bowl, whisk together flour, baking powder, and salt.
In a large mixing bowl, cream together softened butter and sugar until fluffy.
Add eggs one at a time, mixing well after each addition.
Mix in the vanilla extract and then the buttermilk until smooth.
Gradually add the dry ingredients to the wet mixture, mixing until just combined.
Stir in green food coloring and rainbow sprinkles gently.
Pour the batter into the prepared baking sheet, spreading it evenly.
Bake in the preheated oven for 25–30 minutes.
Let the cake cool in the pan for 10 minutes before transferring it to a wire rack.
Frost with buttercream frosting once completely cooled.
Top with additional sprinkles before serving.