Stage 1 — Almond Sponge: Preheat your oven to 180°C (fan 160°C, gas mark 4). Grease a 20cm springform tin and line the base and sides with baking paper.
Add all sponge ingredients — butter, caster sugar, almond extract, eggs, milk, self-raising flour, baking powder, salt, and ground almonds — into one large bowl. Beat with an electric mixer on medium speed for 2 to 3 minutes until smooth and creamy.
Spoon the batter into the prepared tin and smooth to the edges. Bake for 50 to 55 minutes until almost set in the centre. A skewer should come out with just a few moist crumbs. Do not open the oven door during the first 45 minutes.
Stage 2 — Honey Almond Topping: While the sponge bakes, combine butter, caster sugar, honey, and double cream in a small saucepan with a pinch of salt. Heat over medium-low, stirring, until the sugar dissolves.
Raise the heat slightly and bubble for 4 minutes until the mixture turns a soft warm beige. Remove from heat and stir in the flaked almonds until fully coated.
As soon as the sponge passes the skewer test, pour the almond topping evenly over the surface. Work quickly. Return the cake to the oven for a further 7 to 8 minutes until the topping is golden and bubbling.
Remove from the oven and leave in the tin for at least 15 minutes. Then cool completely on a wire rack before cutting or filling.
Stage 3 — Custard Cream Filling: Whisk the custard powder with 150ml milk in a small saucepan until completely smooth. Add 50g of the caster sugar. Stir continuously over low heat until the mixture comes to a boil and thickens. Cook gently for 3 minutes, stirring throughout.
Pour the hot custard into a bowl and press cling film directly onto the surface. Cool completely to room temperature before using. (Can be refrigerated if making ahead.)
In a separate bowl, beat the soft butter with the remaining 50g caster sugar for 3 to 4 minutes until light and creamy. Add the double cream and vanilla, then mix to combine.
Add the cooled custard to the butter mixture in two or three additions, beating well after each one until smooth and silky. If it looks curdled, keep beating — it will come together.
Assembly: Using a long serrated knife, slice the fully cooled cake horizontally into two even layers. Pre-cut the almond-topped half into 12 serving wedges before assembling.
Spread the custard filling generously over the cut face of the bottom sponge layer. Reassemble the pre-cut almond-topped wedges on top, pressing gently. Refrigerate for at least 30 minutes before serving.