Grease the inside of a 6-litre oval or round slow cooker, then line it with foil. Lightly grease the inside of the foil. Turn the slow cooker on to low.
Dot 50g of the butter spread over the bottom of the slow cooker, then scatter over the light brown sugar. Arrange the pears on top. Beat the remaining butter spread with the caster sugar in the bowl of an electric mixer for 5 minutes until light and fluffy.
Add the eggs, one at a time, beating well after each addition. Briefly beat in the vanilla extract. Add the flour, baking powder, ground cinnamon, ground ginger and ground almonds and beat until just combined. Fold in the milk. Pour the mixture over the pears and smooth the top.
Drape a long length of kitchen paper over the slow cooker, not touching the cake, to stop condensation dripping down. Put the lid on the cooker and cook on low for 3 hours, until the cake has set and the sides are brown. Turn off the slow cooker and let the cake rest for 30 minutes with the lid on.
Lift the cake out of the slow cooker using the foil and carefully invert on to a wire rack. Remove the foil. When ready to serve, transfer to a large platter.