Butter an 8" round cake pan and line the bottom with parchment paper.
Preheat your oven to 350℉.
Place the butter and sugar in the bowl of a stand mixture, or in a large bowl if using a hand mixer, and beat on medium speed for about 3-4 minutes. The mixture should be creamy and light in color.
Add the vanilla, whole egg and egg yolk and beat until incorporated.
In a separate bowl whisk together the cake flour, baking powder, baking soda and salt.
Add ⅓ of the flour mixture to the bowl and mix on low speed until just incorporated.
Next add half of the buttermilk and continue to mix on low until combined.
Add another third of the flour mix on low until combined. Add the remaining buttermilk and the remaining flour and mix on low until just combined.
Scrape the bottom and sides of the bowl with a large rubber spatula to insure that all the ingredients have been combined.
Fold in ⅓ cup of jimmie sprinkles.
Place the batter in the prepared pan and smooth the top using an offset spatula.
Bake for 33-35 minutes. The top should be golden brown and spring back when gently pressed.
Remove from the oven and allow the cake to cool in the pan.
Once cool remove the cake from the pan and place on a serving plate.