In a medium bowl, combine the two flours, baking powder, and salt. Whisk together.

In the bowl of your mixer, combine the butter and sugar. Beat on medium-high speed until light and fluffy. Add the eggs, one at a time, mixing well after each addition. Scrape down the bowl as needed. Stir in the vanilla.

With the mixer on low, beat in the dry ingredients in three additions, alternating with the milk. Beat each addition until just incorporated. Pour in the sprinkles and give it a final stir.

Pour the batter into the prepared cake pans. Bake for 30-35 minutes or until a tooth pick inserted in the center comes out clean. Rotate the pans halfway through baking. Transfer the pans to a wire rack and let cool for 20 minutes. Run a knife around the edge of the cakes to loosen the up and then invert them on to the wire rack. Remove and discard the parchment paper. Cool the cakes completely before frosting and level the layers if needed.

Place the chocolate in a heat proof bowl above a pan of simmering water. Heat, stirring occasionally, until the chocolate has melted. Let the chocolate cool to room temperature, about 25-30 minutes. Combine the cocoa powder and boiling water in a small bowl and stir until the cocoa is dissolved.