Make the Cake: Preheat the oven to 350°F. Grease and flour a 9x13 inch rectangular cake pan.
In a large mixing bowl combine the flour, baking soda, and salt.
In the bowl of a standing mixer, combine the butter and sugar and beat until fluffy.
Add the egg whites to the butter mixture one at a time, beating well with each addition.
Mix in the sour cream, and vanilla extract. Beat again and scrape down the bowl.
Add the flour mixture alternately with the whole milk; begin and end with flour. Quickly fold in the confetti sprinkles using a rubber spatula and pour the batter into the prepared pan.
Bake for 25-30 minutes or until a toothpick inserted in the center comes out clean.
Turn the cake out onto a wire rack to cool. Cool completely before frosting.
Make the Frosting: In a mixing bowl beat the butter with an electric mixer until smooth. Add powdered sugar and vanilla, beat until fluffy. Add milk or cream to thin if needed.
Decorate! (See blog post for step pictures and resources.)
Trim the cake so that one piece is 5x13-inches wide. Cover and reserve the other cake piece. Place the 5x13 piece on a large serving platter or cake board.
Cut two notches out of the top edges of the 5x13 cake using a serrated knife to create a ‘bottle top’ shape.
Cover the cake in a crumb coat of frosting; refrigerate. Apply a second smooth coat of frosting to the cake but do not refrigerate. Tidy the edges of the cake plate with a damp paper towel.
Note: At this time you can also frost the top of the reserved cake and cover it in sprinkles. This cake can be cut into squares and served alongside the 'Bottle of Sprinkles Cake’ at your party.
Lay a piece of paper approximately 3-inches wide across the top third of the bottle cake where the label will be. Also cover the ‘bottle top’ portion with paper.
Cover the cake with confetti sprinkles and press them on gently with your hands. Press the sprinkles around the edges of the cake and under the paper where the bottle label is placed. Tilt the plate or cake board and brush the excess confetti away using a pastry brush or your fingers. I left one top corner of the sprinkle jar white so it looks as if the jar is tipped and the sprinkles have fallen to one side.
Remove the papers; some icing will stick to the paper so you’ll need to smooth the icing on the cake using a spatula.
Knead well and roll flat between parchment papers a small portion of blue ready-made fondant. Trim it to the same size as the ‘bottle cap’ portion of the cake. Press a lollipop stick or skewer into the fondant to give the bottle top ridges. Lay the fondant piece on the cake and gently press so that it adheres to the frosting.
Measure the bottle’s label area; roll and cut a piece of white fondant to size. Also cut a piece of white wafer paper to size. Lay the fondant piece on the exposed cake.
Use 1" brush letter stencils and a fine-tipped food color marker to draw ‘Sprinkles’ onto the wafer paper. Don’t press the pen too hard on the paper; lightly outline the letters and then quickly color them in. Attach the paper to the fondant using dots of corn syrup on its underside (one dot at each corner should do it!).
Peel off large candy buttons from their paper. Attach the buttons in alternating colors using corn syrup on their undersides; place them around the edge of the ‘Sprinkles’ label in marquee fashion.
Serve to delighted guests! Store cake loosely covered with plastic wrap at room temperature.