- With the rack in the middle position, preheat the oven to 180°C (350°F). Butter a 20-cm (8-inch) springform pan. Line the bottom with parchment paper.
- In a bowl, combine the flour, baking powder and salt. Set aside.
- In another bowl, cream the butter, sugar and vanilla using an electric mixer.
- Add the eggs 1 at a time, beating until the mixture is smooth. With the mixer on low, add 375 ml (1 1/2 cups) strawberries. Add the dry ingredients, alternating with the milk.
- Scrape the batter into the springform pan. Bake until a toothpick inserted in the centre comes out clean, about 1 hour.
- Let cool partially, unmould and finish cooling on a rack.
- Break the egg into a large bowl. Set aside.
- In a saucepan, bring the jam and sugar to a boil while stirring. Simmer for 2 minutes.
- With an electric mixer on medium speed, beat the egg until frothy. Slowly add the hot jam, without pouring it directly on the beaters. Beat until the mixture is completely cooled, about 15 minutes.
- Add the butter 1 or 2 pieces at a time, beating until the mixture is very creamy.
- On a clean surface, slice the cake in half horizontally, making 2 layers. Spread buttercream on top of each layer. Stack 1 layer on the other. Decorate the top with the remaining strawberries.