Preheat the oven to 350°F (180°C). Grease two 8-inch round cake pans and line the bottoms with parchment paper.
In a large bowl, beat the softened butter and caster sugar with a hand mixer or stand mixer on medium speed until very pale and fluffy, 3–5 minutes.
Beat in the eggs one at a time, mixing well after each addition. If the mixture looks slightly curdled, beat in a spoonful of the measured flour to bring it back together.
In a separate bowl, sift together the all-purpose flour and baking powder. Gently fold the dry ingredients into the batter in two additions until just combined and no dry patches remain.
Divide the batter evenly between the prepared pans and level the tops.
Bake until well risen, golden, and the tops spring back when lightly pressed, 20–25 minutes. A skewer inserted in the center should come out clean.
Cool the cakes in the pans for 10 minutes, then turn out onto a wire rack, peel off the parchment, and let cool completely, 45–60 minutes.
While the cakes cool, whip the heavy cream with the powdered sugar to soft peaks, 1–2 minutes; keep chilled.
Place one cake layer on a serving plate. Spread the raspberry jam evenly over the top, then spread the whipped cream over the jam.
Set the second cake layer on top. Dust the top with caster sugar. Slice and serve.