In a stand mixer or large bowl, beat the softened butter and sugar on medium speed until light and fluffy. Add the egg, vanilla extract, and milk, and continue beating until smooth.
In a separate bowl, sift together the flour, baking powder, and salt. Gradually add the dry ingredients into the wet mixture, blending just until combined.
In another bowl, whisk the instant pistachio pudding mix with 3 cups of whole milk for about 2 minutes until creamy. Let it sit for 5 minutes to thicken.
Line a 5-quart slow cooker with a baking bag or parchment paper. Pour the pistachio pudding into the slow cooker and smooth it out.
Carefully layer the vanilla cake batter over the pudding. Evenly sprinkle the chopped maraschino cherries over the top.
Cover and cook on LOW for 4 hours, or until the edges are golden and set. Once done, let the slow cooker insert cool for 10 minutes.
Carefully invert the insert onto a baking sheet or serving platter. Gently peel away the baking bag while the cake is still warm.
While the cake is hot, sprinkle the white chocolate chips, semi-sweet chocolate chips, and chopped pistachios over the top.
Serve warm, optionally with a scoop of vanilla ice cream.