In a small saucepan, whisk together flour, milk, vanilla extract, and salt until smooth. Heat over medium-high, whisking constantly, until mixture becomes thick and resembles pudding, 3 to 4 minutes. Remove from heat; transfer to a bowl. Cover surface with plastic wrap and let cool completely, about 30 minutes.

In a stand mixer fitted with the paddle attachment, beat butter with sugar and vanilla seeds on medium speed, scraping down sides as necessary, until light and fluffy, about 2 minutes. Add milk mixture and continue to beat until frosting is light and fluffy and looks like whipped cream, 2 to 3 minutes. Use immediately.