Preheat oven to 350℉ (180℃). Grease and flour a 9x9-inch (22.5 cm²) square baking pan, or grease a 10-inch (25 cm) round springform pan and line the bottom with a parchment circle cut to fit. If using a springform pan, use the bottom disk upside-down in the pan, so the baked cake slides off the bottom easily once the sides are released.
Put the butter, flour, almond flour, sugar, baking powder, salt, milk, eggs, vanilla, and almond extract into a mixing bowl. Beat with the electric beater or with a wooden spoon until smooth.
Stir in the saskatoon berries with a spatula.
Scrape the batter into the prepared pan and smooth the top, Sprinkle evenly with the sliced almonds.
Bake for 45 - 50 minutes, until a toothpick inserted in the middle comes out clean.
Cool the pan for 5 minutes in the pan, then poke the cake all over with a skewer, making the holes about 1 inch (2.5cm) apart.
In a small saucepan, heat the saskatoon jelly and lemon juice until the jelly is melted and liquid. Spoon the hot jelly mixture evenly over the top of the warm cake, letting each spoonful absorb into the cake before you add another one. Make sure to cover the whole cake with the jelly and make sure all the almond slices are coated.
Remove the sides of the pan if using a springform pan and allow the cake to cool, then slip a thin metal flipper under the cake and carefully slide it onto a cake plate, helping it along with the flipper. If you made the cake in a square pan, you can leave it in the pan to serve it.
Serve with lightly sweetened whipped cream or serve the cake warm with ice cream. The cake will keep, covered and refrigerated, for up to 5 days.
Notes
You can use frozen saskatoon berries. Either measure them by weight, or use heaping cupfuls, as they don't fit as compactly into a measuring cup when frozen.