In a standing mixer add the butter and sugar and mix until combined. Once combined, whip on high for 2 minutes, pausing to scrape down the sides of the mixing bowl every 30 seconds. The mixture should be lighter in color and texture when finished.
Add the eggs, milk, sour cream and vanilla. Stir until combined.
In a separate mixing bowl combine flour, baking powder, salt and cornstarch.
Turn the standing mixer on low and slowly pour in the dry ingredients. Mix until the batter is all combined.
Once combined, turn the mixer on medium speed for about 20 seconds until the batter is smooth.
Pour the cake batter into two greased and parchment paper lined cookie sheets.
Bake the cake for 20 minutes, remove and let cool in the pan.
Once cooled, wrap the cookie sheets in tinfoil and place in the freezer for about 30 minutes to chill.
Frosting
In a standing mixer add the butter and whip until it is softened
Add the powdered sugar and mix until combined
Add the cream and vanilla extract, whip the frosting for 2 full minutes until light and fluffy
Frost the cake when it is chilled
Assemble
Once the cake is chilled, use the small pastry rings to cut out cake circles
When all the cake is cut, take the cake acetate paper and measure a circle the same size as the pastry ring. Tape the acetate together to make a mold the same width as the pastry ring.
Scoop your frosting into a piping bag, snip the end of the piping bag off, you do not need a tip for this.
Take the 4" cake round and smear frosting on top. Place the acetate form you just made on top of the cake round. Take 1 of the cake circles and press it in the form all the way down to the cake round so that it's glued on the frosting. Pipe a layer of frosting on top of the cake. Repeat twice until the cake is 3 layers tall.
Place the mini cake with the acetate form in the freezer for about 30 minutes to firm up.
Once chilled, remove from the freezer and cut off the acetate form.