For the Mint Simple Syrup:

1. Bring mint leaves, sugar, and water to a boil in a small pot, then remove from heat and let steep 15 minutes.
2. Strain out mint leaves.

For the Cake:

1. Preheat oven to 350 degrees F. Spray an 8-inch round pan with cooking spray, then line with parchment paper. Spray as much as is needed to allow the parchment paper to adhere to the pan.
2. In a large bowl, combine flour, sugar, baking soda, and salt.
3. In a small bowl, whisk together water, olive oil, lemon juice and vanilla.
4. Pour wet ingredients into the large bowl. Whisk or stir just into no lumps remain.
5. Brush the parchment paper on the bottom of the pan with mint simple syrup.
6. Add frozen berries to the pan, making sure the berries are evenly covering the bottom surface in a single layer. Brush the tops of the berries with more simple syrup.
7. Gently pour prepared batter over the berries into the pan.
8. Bake until a toothpick comes out clean or the center of the cake springs back when lightly touched, about 45-60 minutes.
9. Let cake cool on a rack, about 30 minutes, then carefully flip the cake out of the pan and directly onto a serving plate.
10. Once cake is no longer warm to the touch, dust with powdered sugar and finish with a sprig of mint leaves, if desired.