Preheat oven to 350 degrees Fahrenheit and line cupcake or muffin tins with 18 paper liners of your choice.
Next, start preparing the cupcake batter by adding the butter and white sugar to the bowl of a stand mixer. Beat the ingredients together on medium-high speed for several minutes, until the mixture becomes light, fluffy and pale in colour.
Beat in each of the 5 eggs, one at a time, on medium speed, stopping between each addition to scrape down the sides and the bottom of the bowl to make sure everything is incorporated evenly.
Measure the flour, baking powder and sea salt into a separate bowl and set it aside.
Measure out the milk and add the vanilla to it.
Add the dry ingredients and wet ingredients alternately, starting and ending with the dry ingredients. Add the ingredients on the lowest speed and be careful not to over mix...only let the mixer run until the ingredients are just combined. Add the last addition of dry ingredients by hand with a rubber spatula just to prevent over mixing, which leads to a tough and dense crumb.
Divide the batter evenly between the 18 paper liners, filling each about 3/4 full. Measure out one teaspoon of strawberry jam onto the top of each well of batter. Using a butter knife, cut into the jam and swirl it around in the top half of the batter.
Place pans in the oven and bake at 350 for 18-20 minutes. Remove pan from oven and wait 2 minutes, then remove the cupcakes from the pan and let them cool completely on a wire rack.
While you're waiting for the cupcakes to cool you can begin to prepare the frosting.