1Preheat oven to 350F. Grease and flour three 6" cake rounds and line with parchment.
2In a medium bowl, whisk flour, 1/2 cup strawberry powder, baking powder, and salt. Sift to ensure it's well combined. Set aside.
3Using a stand mixer fitted with a paddle attachment, cream butter and sugar on med-high until pale and fluffy (approx. 3 min.). Reduce speed and add eggs one at a time fully incorporating after each addition. Add vanilla.
4Alternate adding flour mixture and milk, beginning and ending with flour (3 additions of flour and 2 of milk). Fully incorporating after each addition.
5Bake for 35-40 minutes or until a toothpick inserted into the center comes out mostly clean.
6Place cakes on wire rack to cool for 10mins then turn out onto wire rack to cool completely.
1In a stand mixer with the paddle attachment, cream the butter. Start the mixer at the lowest speed, then gradually increase the speed, using a rubber spatula to scrape the bowl as needed, until the butter is light in color, is perfectly smooth, and makes a slapping sound as it hits the sides of the bowl. This should only take 30 seconds, but if the butter is cool, it can take a couple of minutes.
2Add the sugar, 1 cup (about 110 g) at a time, and mix at the lowest speed until it’s fully incorporated before adding the next cup. When all the sugar has been added, scrape the paddle and the bottom and sides of the bowl. Add the vanilla and salt, and beat at low speed for 15 seconds. Increase the speed to as high as you can without making a mess, and whip the buttercream until it is perfectly smooth, creamy, and light, about 5 minutes. Stop the mixer once or twice to scrape the bowl and paddle, then continue beating. Add strawberry rhubarb jam and mix. At rest the buttercream will appear to soften, then it will stiffen and increase in volume.
3Use right away, or store in an airtight container for up to a week in the refrigerator. When you’re ready to use it, let the buttercream come to room temperature, then put it back in the stand mixer and use the paddle attachment to beat it until it’s creamy and stiff again.