Prepare your strawberry jam. In a medium saucepan, toss together the strawberries, sugar, and lemon juice. Bring to a boil over medium heat then reduce to a simmer, mashing the berries with a wooden spoon against the edge of the pot as it cooks. Cook for about 10 minutes until it thickens to coat the back of a spoon and the berries are well broken down. Whisk the corn starch into the water to make a slurry and while stirring the pot, pour it into the berries. Cook for 1 more minute then transfer to a clean bowl and chill until cooled to room temperature, at least 30 minutes (speed this up in the freezer, stirring every few minutes!).
While the filling cools, toss the 2 teaspoons of sugar into the heart-shaped strawberries and chill to macerate.
Once the filling is cooled, prepare the cake. Preheat the oven to 350°F and line an 8-inch square baking pan with parchment paper.
In medium bowl, whisk together flour, baking powder, baking soda, salt, and cinnamon. Set aside.
In a large mixing bowl, whisk together dark brown sugar, sour cream, peanut butter, eggs, oil, water, and vanilla until completely smooth. Dump the dry ingredients into the wet ingredients and stir with a rubber spatula until smooth.
Pour batter into prepared pan. Dollop 6 tablespoons of jam over the top and use a knife to swirl it into the top of the cake. Reserve any remaining jam for later use. Bake for 40-43 minutes until a tester comes out clean or with just a few crumbs.
Let cool in the pan for 10 minutes before transferring to a wire rack to completely before frosting.
Once cool, prepare the peanut butter buttercream. In a large bowl, cream together the butter and peanut butter until completely smooth with an electric hand mixer. Add the powdered sugar in 3 batches, mixing until combined. Add the vanilla and salt and mix until smooth. Add up to 2 teaspoons of milk to loosen the frosting as needed to make it spreadable (some peanut butters will be more moist than others, so you may not need any!).
Swirl the frosting over the cake and top with macerated heart-shaped strawberries.