Thickly butter a 6-8 cup gugelhupf pan (or other decorative tube pan).
Make the sponge: warm the milk over low heat in a small saucepan until it is just lukewarm, about 100°.
Transfer milk to a small bowl; whisk in yeast.
Stir in the flour and cover the bowl with plastic wrap; let the sponge rise until very puffy, about 20 minutes.
Meanwhile, stir together the raisins and rum; let raisins soak in rum while you prepare the dough.
Make the dough: use an electric stand mixer with paddle attachment—beat the butter with the sugar and salt on medium speed until soft and light, about 2-3 minutes.
Beat in the lemon zest and vanilla.
Beat in the egg yolks, one at a time, beating smooth after each addition.
Stop mixer and scrape the sponge into the bowl; beat on lowest speed until incorporated, about 1 minute.
Drain raisins well, reserving the rum; beat the rum into the dough, then beat in the flour.
Beat the dough for 2 minutes on lowest speed, then stop the mixer and let dough rest for 10 minutes.
Beat the dough on medium speed until it is smooth and elastic, about 2 additional minutes.
Decrease speed to lowest and beat in the raisins and the chopped almonds.
Scrape the dough into a buttered bowl and turn it over so that the top is buttered.
Cover bowl with plastic wrap and let dough rise until it just begins to puff, about 20 minutes.
Prepare mold: scatter the sliced almonds all over the buttered inside of the mold, rotating the mold to cover the buttered surface evenly with almonds.
Invert to mold to remove any of the almonds that have not stuck to the butter.
Using a large rubber spatula , carefully scrape the dough into prepared mold—try to avoid disturbing the sliced almonds.
Cover the mold with a towel or piece of buttered plastic wrap and let the dough rise until it is doubled (if using a 6-cup mold, the risen dough should reach the top of the mold; if you are using an 8-cup mold, the risen dough should reach about ½ inch short of the top).
About 15 minutes before the gugelhupf is risen, set a rack in the middle of the oven and preheat to 375°.
Bake the gugelhupf until it is well risen and deep golden, about 40-45 minutes; pick should come out clean.
Cool gugelhupf in pan on a rack for 10 minutes, then invert it onto the rack, remove the mold and cool completely.
Serve: use a sharp serrated knife to cut into slices; storage: keep wrapped in plastic.