Spray a 9x13 rectangle pan with nonstick spray and preheat the oven to 350 degrees.
In one bowl, combine flour and baking soda. Whisk together and set aside.
In another bowl, beat butter on medium-high speed with an electric mixer until smooth and creamy. Add sugar and mix until combined and fluffy. Next add the egg, egg white, and vanilla, and mix well on medium speed. Then add the strawberry sauce and mix until combined.
Next add half of the flour mixture and half of the buttermilk. Mix on low until just combined. Add the second half of the flour and buttermilk and mix until completely combined.
Pour the batter into the prepared pan and bake for 20 minutes. Let the cake cool in the pan for 10 minutes, then let it cool completely on a wire rack.
When the cake is cool, transfer it to a sheet of parchment paper and cut out the circles, which will become the small cake layers. I used a large cup for this, but you can use whatever you have. My cake layers ended up being about 4 inches in diameter.