In a medium saucepan, bring 4 cups of water to a boil.

In a heavy bottomed pot, combine water and sugar. Place over medium-high heat and bring to a boil or 212 degrees F.

In a medium bowl, prepare the ice bath by filling it with ice and covering them with water.

Boil the lemon slices for 2 minutes, then transfer them to the ice bath.

Once the sugar syrup has reached 212 degrees F, carefully add the lemon slices, turn the heat up to high, and bring the syrup up to 225 degrees F. Do not move them around too much, but keep them from sticking to the sides of the pan. This can take about 20-30 minutes.

Transfer the lemon slices to a cooling rack over a parchment lined baking sheet and allow them to cool while you make the cake batter.

Preheat the oven to 325 degrees F. Line a 9" cake pan with parchment paper.

In a medium bowl, combine the flour, baking powder, and salt. Set aside.

In a large bowl, combine the sugar and lemon zest. Rub them together with your fingers until the it resembles wet sand and the zest smells aromatic.

Add the olive oil, lemon juice, eggs, and vanilla. Use an electric hand mixer or whisk to mix until well combined.

Add the dry ingredients and mix until fully combined.

Arrange the candied lemon slices in the cake pan, overlapping them slightly.

Very slowly pour the cake batter into the pan, careful not to disturb the lemon slices.

Bake the cake for 45-50 minutes or until lightly golden on top and a toothpick inserted comes out clean.

Once the cake comes out of the oven, use a knife to loosen the sides from the pan. Place a large plate on top and flip the cake over. Remove the pan, then gently peel back the parchment paper.

Allow the cake to cool completely, then use a serrated knife to cut individual slices, and enjoy!